Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2012	Dec. 31, 2013
Investment Securities [Line Items]
Amortized Cost	$ 1,348,063		$ 1,350,466
Gross Unrealized Gains	15,361	15,565
Gross Unrealized Losses	(30,650)	(47,666)
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
Gross Unrealized Gains	2,617	2,624
Gross Unrealized Losses	10	(59)
Securities available for sale, at fair value	24,387		25,204
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Residential [Member]
Investment Securities [Line Items]
Amortized Cost	987,530		1,009,519
Gross Unrealized Gains	11,642	12,531
Gross Unrealized Losses	16,414	(27,163)
Securities available for sale, at fair value	982,758		994,887
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	79		104
Gross Unrealized Gains	2	1
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	81		105
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	269,177		267,971
Gross Unrealized Gains	0	81
Gross Unrealized Losses	1,460	(1,927)
Securities available for sale, at fair value	267,717		266,125
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Amortized Cost	19,421		80
Gross Unrealized Gains	251	0
Gross Unrealized Losses	3	0
Securities available for sale, at fair value	19,669		80
Corporate Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	6,685		6,693
Gross Unrealized Gains	601	328
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	7,286		7,021
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	41,971		42,040
Gross Unrealized Gains	248	0
Gross Unrealized Losses	(12,763)	(18,517)
Securities available for sale, at fair value	29,456		23,523
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,346,643		1,349,046
Gross Unrealized Gains	15,361	15,565
Gross Unrealized Losses	(30,650)	(47,666)
Securities available for sale, at fair value	1,331,354		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