Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2012	Dec. 31, 2013
Investment Securities [Line Items]
Amortized Cost	$ 1,357,315		$ 1,350,466
Gross Unrealized Gains	18,717	15,565
Gross Unrealized Losses	(28,421)	(47,666)
Securities available for sale, at fair value	1,347,611		1,318,365
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	20,686		22,639
Gross Unrealized Gains	2,617	2,624
Gross Unrealized Losses	0	(59)
Securities available for sale, at fair value	23,303		25,204
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Residential [Member]
Investment Securities [Line Items]
Amortized Cost	1,001,669		1,009,519
Gross Unrealized Gains	15,257	12,531
Gross Unrealized Losses	13,807	(27,163)
Securities available for sale, at fair value	1,003,119		994,887
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	88		104
Gross Unrealized Gains	2	1
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	90		105
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	269,175		267,971
Gross Unrealized Gains	15	81
Gross Unrealized Losses	803	(1,927)
Securities available for sale, at fair value	268,387		266,125
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Amortized Cost	15,738		80
Gross Unrealized Gains	156	0
Gross Unrealized Losses	5	0
Securities available for sale, at fair value	15,889		80
Corporate Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	6,688		6,693
Gross Unrealized Gains	561	328
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	7,249		7,021
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	41,851		42,040
Gross Unrealized Gains	109	0
Gross Unrealized Losses	(13,806)	(18,517)
Securities available for sale, at fair value	28,154		23,523
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,355,895		1,349,046
Gross Unrealized Gains	18,717	15,565
Gross Unrealized Losses	(28,421)	(47,666)
Securities available for sale, at fair value	1,346,191		1,316,945
Equity Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,420		1,420
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	$ 1,420		$ 1,420