Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investment Securities [Line Items]
Amortized Cost	$ 1,045,850	$ 1,317,318
Gross Unrealized Gains	17,069	16,921
Gross Unrealized Losses	(12,186)	(24,420)
Securities available for sale, at fair value	1,050,733	1,309,819
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	20,902	23,344
Gross Unrealized Gains	2,379	2,595
Gross Unrealized Losses	0	(3)
Securities available for sale, at fair value	23,281	25,936
Residential Mortgage Backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	816,179	947,635
Gross Unrealized Gains	12,819	13,076
Gross Unrealized Losses	4,846	(9,830)
Securities available for sale, at fair value	824,152	950,881
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	37	72
Gross Unrealized Gains	1	2
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	38	74
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	135,853	269,181
Gross Unrealized Gains	19	4
Gross Unrealized Losses	91	(1,308)
Securities available for sale, at fair value	135,781	267,877
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Amortized Cost	27,064	27,058
Gross Unrealized Gains	397	362
Gross Unrealized Losses	12	(43)
Securities available for sale, at fair value	27,449	27,377
Corporate Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,895	6,682
Gross Unrealized Gains	438	573
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	2,333	7,255
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	42,000	41,926
Gross Unrealized Gains	1,016	309
Gross Unrealized Losses	(7,237)	(13,236)
Securities available for sale, at fair value	35,779	28,999
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,043,930	1,315,898
Gross Unrealized Gains	17,069	16,921
Gross Unrealized Losses	(12,186)	(24,420)
Securities available for sale, at fair value	1,048,813	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