Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Parenthetical) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investment Securities [Line Items]
Amortized Cost	$ 1,045,850	$ 1,317,318
Securities available for sale, at fair value	1,050,733	1,309,819
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	20,902	23,344
Securities available for sale, at fair value	23,281	$ 25,936
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	816,200
Securities available for sale, at fair value	$ 824,200