Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Additional Information (Detail)

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Fair Value Measurements - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Tax credit to be received 2 years      
Percentage of decline in bond value 1.60%      
Loans Held for Sale $ 8,306,000 $ 6,043,000 $ 55,993,000 $ 22,587,000
Zero Coupon Municipal Bond [Member]
       
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Zero coupon tax credit municipal bond, recorded at estimated fair value 779,000 774,000    
Significant Observable Inputs With No Active Market With Identical Characteristics [Member] | Nonrecurring Basis [Member] | Nonperforming loans held for sale [Member]
       
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Loans Held for Sale 484,000 2,300,000    
Significant Observable Inputs With No Active Market With Identical Characteristics [Member] | Small Business Administration Loans Held for Sale [Member] | Nonrecurring Basis [Member]
       
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Loans Held for Sale $ 7,800,000 $ 3,700,000