Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Investment Securities [Line Items]
Amortized Cost	$ 1,091,191	$ 1,317,318
Gross Unrealized Gains	14,953	16,921
Gross Unrealized Losses	(16,419)	(24,420)
Securities available for sale, at fair value	1,089,725	1,309,819
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	21,846	23,344
Gross Unrealized Gains	2,516	2,595
Gross Unrealized Losses	24	(3)
Securities available for sale, at fair value	24,338	25,936
Residential Mortgage Backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	860,816	947,635
Gross Unrealized Gains	10,799	13,076
Gross Unrealized Losses	8,715	(9,830)
Securities available for sale, at fair value	862,900	950,881
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	58	72
Gross Unrealized Gains	1	2
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	59	74
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	135,852	269,181
Gross Unrealized Gains	5	4
Gross Unrealized Losses	228	(1,308)
Securities available for sale, at fair value	135,629	267,877
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Amortized Cost	27,062	27,058
Gross Unrealized Gains	176	362
Gross Unrealized Losses	173	(43)
Securities available for sale, at fair value	27,065	27,377
Corporate Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,892	6,682
Gross Unrealized Gains	401	573
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	2,293	7,255
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	41,745	41,926
Gross Unrealized Gains	1,055	309
Gross Unrealized Losses	(7,279)	(13,236)
Securities available for sale, at fair value	35,521	28,999
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,089,271	1,315,898
Gross Unrealized Gains	14,953	16,921
Gross Unrealized Losses	(16,419)	(24,420)
Securities available for sale, at fair value	1,087,805	1,308,399
Equity Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,920	1,420
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	$ 1,920	$ 1,420