Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment Securities [Line Items]
Amortized Cost	$ 1,266,915	$ 1,317,318
Gross Unrealized Gains	20,344	16,921
Gross Unrealized Losses	(17,974)	(24,420)
Securities available for sale, at fair value	1,269,285us-gaap_AvailableForSaleSecurities	1,309,819us-gaap_AvailableForSaleSecurities
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	22,578	23,344
Gross Unrealized Gains	2,835	2,595
Gross Unrealized Losses	0	(3)
Securities available for sale, at fair value	25,413	25,936
Residential Mortgage Backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	905,895	947,635
Gross Unrealized Gains	15,759	13,076
Gross Unrealized Losses	6,402	(9,830)
Securities available for sale, at fair value	915,252	950,881
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	65	72
Gross Unrealized Gains	1	2
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	66	74
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	260,849	269,181
Gross Unrealized Gains	180	4
Gross Unrealized Losses	330	(1,308)
Securities available for sale, at fair value	260,699	267,877
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Amortized Cost	27,060	27,058
Gross Unrealized Gains	494	362
Gross Unrealized Losses	5	(43)
Securities available for sale, at fair value	27,549	27,377
Corporate Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	6,680	6,682
Gross Unrealized Gains	572	573
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	7,252	7,255
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	41,868	41,926
Gross Unrealized Gains	503	309
Gross Unrealized Losses	(11,237)	(13,236)
Securities available for sale, at fair value	31,134	28,999
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,264,995	1,315,898
Gross Unrealized Gains	20,344	16,921
Gross Unrealized Losses	(17,974)	(24,420)
Securities available for sale, at fair value	1,267,365	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

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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-Publisher FASB

-Name Accounting Standards Codification

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