Securities - Schedule of Amortized Cost and Fair Value of Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	$ 1,513,204us-gaap_HeldToMaturitySecurities	$ 1,453,355us-gaap_HeldToMaturitySecurities
Securities Held To Maturity, Gross Unrealized Gains	28,454	23,145
Securities Held To Maturity, Gross Unrealized Losses	(5,209)	(8,242)
Securities held to maturity, fair value	1,536,449	1,468,258
U.S. Treasury [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	500	502
Securities Held To Maturity, Gross Unrealized Gains	182	168
Securities held to maturity, fair value	682	670
U.S. Government-Sponsored Entities [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	146,578	101,602
Securities Held To Maturity, Gross Unrealized Gains	1,731	885
Securities Held To Maturity, Gross Unrealized Losses	(149)	(524)
Securities held to maturity, fair value	148,160	101,963
Agency Mortgage-Backed Securities [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	674,101	677,169
Securities Held To Maturity, Gross Unrealized Gains	19,130	16,712
Securities Held To Maturity, Gross Unrealized Losses	(20)	(346)
Securities held to maturity, fair value	693,211	693,535
Agency Collateralized Mortgage Obligations [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	505,859	501,965
Securities Held To Maturity, Gross Unrealized Gains	3,633	1,858
Securities Held To Maturity, Gross Unrealized Losses	(4,884)	(7,329)
Securities held to maturity, fair value	504,608	496,494
Non-Agency Collateralized Mortgage Obligations [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	3,840	4,285
Securities Held To Maturity, Gross Unrealized Gains	21	28
Securities held to maturity, fair value	3,861	4,313
Commercial Mortgage-Backed Securities [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	17,484	17,560
Securities Held To Maturity, Gross Unrealized Gains	385	179
Securities held to maturity, fair value	17,869	17,739
States of the U.S. and Political Subdivisions [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	164,842	150,272
Securities Held To Maturity, Gross Unrealized Gains	3,372	3,315
Securities Held To Maturity, Gross Unrealized Losses	(156)	(43)
Securities held to maturity, fair value	$ 168,058	$ 153,544

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (f)

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Section 25

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

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-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.6)

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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