Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investment Securities [Line Items]
Amortized Cost	$ 1,348,063	$ 1,350,466
Securities available for sale, at fair value	1,332,774	1,318,365
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	21,780	22,639
Securities available for sale, at fair value	24,387	25,204
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	987,600
Securities available for sale, at fair value	$ 982,800