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Investment securities available- for-sale - Additional Information (Detail) - Entity [Domain] - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]          
Proceeds from the sale of investment securities available-for-sale     $ 70,005,000 $ 0  
Available-for-sale Securities, Gross Unrealized Losses     55,385,000   $ 65,502,000
Available-for-sale Securities, Amortized Cost Basis $ 5,568,667,000   5,568,667,000   5,307,250,000
US Government Sponsored Enterprises Debt Securities Member          
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale Securities, Gross Unrealized Losses     4,691,000   12,391,000
Available-for-sale Securities, Amortized Cost Basis 1,112,531,000   1,112,531,000   $ 1,735,758,000
Segment, Continuing Operations [Member]          
Schedule of Available-for-sale Securities [Line Items]          
Other Than Temporary Impairment Losses Investments Portion Recognized In Earnings Net (14,445,000) $ 0 (14,445,000) 0  
Net Gain Loss And Valuation Adjustments On Investment Securities 5,000 $ 0 5,000 $ 0  
Available For Sale Securities Member | AFS securities sold [Member] | Segment, Continuing Operations [Member] | US Government Sponsored Enterprises Debt Securities Member          
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale Securities, Amortized Cost Basis $ 70,000,000   $ 70,000,000