Quarterly report pursuant to Section 13 or 15(d)

Investment Securities - Additional Information (Detail)

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Investment Securities - Additional Information (Detail) (USD $)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2013
Investment Securities [Line Items]          
Other-than-temporary impairment charge     $ 0   $ 0
Impairment charges on securities in continuous unrealized loss position for 12 months or longer 0   0   0
Net realized gain (loss) on redemption or sale of investment securities 67,000 611,000 1,852,000 923,000  
Recognized in earnings resulting from the redemption of investment securities 23,000 899,000 498,000 2,400,000  
Investment securities available-for-sale, carrying values 1,128,624,000   1,128,624,000   530,926,000
Collateralized mortgage obligations [Member]
         
Investment Securities [Line Items]          
Investment securities available-for-sale, carrying values 196,211,000   196,211,000   127,693,000
Mortgage-backed securities [Member]
         
Investment Securities [Line Items]          
Investment securities available-for-sale, carrying values 584,157,000   584,157,000   217,059,000
Mortgage-backed securities [Member] | Collateralized mortgage obligations [Member]
         
Investment Securities [Line Items]          
contractual maturities     2064    
Securities Pledged as Collateral [Member]
         
Investment Securities [Line Items]          
Investment securities available-for-sale, carrying values $ 79,900,000   $ 79,900,000   $ 47,600,000