Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail)

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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale $ 578,804 $ 516,964
Collateralized mortgage obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 117,768 76,451
U.S. government agency securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 7,414 7,441
Other securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 22,386 22,394
Recurring Basis [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 578,804 516,964
Recurring Basis [Member] | SBA loan pools securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   4,146
Recurring Basis [Member] | Municipal bonds-tax exempt [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 127,475 158,030
Recurring Basis [Member] | Municipal bonds-taxable [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   13,701
Recurring Basis [Member] | Collateralized mortgage obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 117,768 76,451
Recurring Basis [Member] | Mortgage-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 303,609 229,630
Recurring Basis [Member] | U.S. government agency securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 7,414 7,441
Recurring Basis [Member] | Corporate bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   5,015
Recurring Basis [Member] | U.S treasury securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 152 156
Recurring Basis [Member] | Other securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 22,386 22,394
Recurring Basis [Member] | Quoted Prices in Active Markets for Identical Assets [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 22,538 22,550
Recurring Basis [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | SBA loan pools securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   0
Recurring Basis [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | Municipal bonds-tax exempt [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | Municipal bonds-taxable [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   0
Recurring Basis [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | Collateralized mortgage obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | Mortgage-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | U.S. government agency securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | Corporate bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   0
Recurring Basis [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | U.S treasury securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 152 156
Recurring Basis [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | Other securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 22,386 22,394
Recurring Basis [Member] | Significant Observable Inputs with No Active Market with Identical Characteristics [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 556,266 494,414
Recurring Basis [Member] | Significant Observable Inputs with No Active Market with Identical Characteristics [Member] | SBA loan pools securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   4,146
Recurring Basis [Member] | Significant Observable Inputs with No Active Market with Identical Characteristics [Member] | Municipal bonds-tax exempt [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 127,475 158,030
Recurring Basis [Member] | Significant Observable Inputs with No Active Market with Identical Characteristics [Member] | Municipal bonds-taxable [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   13,701
Recurring Basis [Member] | Significant Observable Inputs with No Active Market with Identical Characteristics [Member] | Collateralized mortgage obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 117,768 76,451
Recurring Basis [Member] | Significant Observable Inputs with No Active Market with Identical Characteristics [Member] | Mortgage-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 303,609 229,630
Recurring Basis [Member] | Significant Observable Inputs with No Active Market with Identical Characteristics [Member] | U.S. government agency securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 7,414 7,441
Recurring Basis [Member] | Significant Observable Inputs with No Active Market with Identical Characteristics [Member] | Corporate bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   5,015
Recurring Basis [Member] | Significant Observable Inputs with No Active Market with Identical Characteristics [Member] | U.S treasury securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Significant Observable Inputs with No Active Market with Identical Characteristics [Member] | Other securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Significant Unobservable Inputs [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Significant Unobservable Inputs [Member] | SBA loan pools securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   0
Recurring Basis [Member] | Significant Unobservable Inputs [Member] | Municipal bonds-tax exempt [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Significant Unobservable Inputs [Member] | Municipal bonds-taxable [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   0
Recurring Basis [Member] | Significant Unobservable Inputs [Member] | Collateralized mortgage obligations [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Significant Unobservable Inputs [Member] | Mortgage-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Significant Unobservable Inputs [Member] | U.S. government agency securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Significant Unobservable Inputs [Member] | Corporate bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale   0
Recurring Basis [Member] | Significant Unobservable Inputs [Member] | U.S treasury securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale 0 0
Recurring Basis [Member] | Significant Unobservable Inputs [Member] | Other securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total securities available for sale $ 0 $ 0