Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment Securities [Line Items]
Amortized Cost	$ 1,266,915	$ 1,317,318
Securities available for sale, at fair value	1,269,285	1,309,819
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	22,578	23,344
Securities available for sale, at fair value	25,413	25,936
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	906,000
Securities available for sale, at fair value	$ 915,300