Securities - Schedule of Amortized Cost and Fair Value of Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	$ 1,453,355us-gaap_HeldToMaturitySecurities	$ 1,199,169us-gaap_HeldToMaturitySecurities	$ 1,106,563us-gaap_HeldToMaturitySecurities
Securities Held To Maturity, Gross Unrealized Gains	23,145	15,484	36,785
Securities Held To Maturity, Gross Unrealized Losses	(8,242)	(25,090)	(135)
Securities held to maturity, fair value	1,468,258	1,189,563	1,143,213
U.S. Treasury [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	502	503	503
Securities Held To Maturity, Gross Unrealized Gains	168	99	188
Securities held to maturity, fair value	670	602	691
U.S. Government-Sponsored Entities [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	101,602	43,322	28,731
Securities Held To Maturity, Gross Unrealized Gains	885	180	280
Securities Held To Maturity, Gross Unrealized Losses	(524)	(1,151)	(99)
Securities held to maturity, fair value	101,963	42,351	28,912
Agency Mortgage-Backed Securities [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	677,169	628,681	780,022
Securities Held To Maturity, Gross Unrealized Gains	16,712	12,281	28,783
Securities Held To Maturity, Gross Unrealized Losses	(346)	(6,032)	(1)
Securities held to maturity, fair value	693,535	634,930	808,804
Agency Collateralized Mortgage Obligations [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	501,965	385,408	133,976
Securities Held To Maturity, Gross Unrealized Gains	1,858	764	1,266
Securities Held To Maturity, Gross Unrealized Losses	(7,329)	(15,844)
Securities held to maturity, fair value	496,494	370,328	135,242
Non-Agency Collateralized Mortgage Obligations [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	4,285	6,852	14,082
Securities Held To Maturity, Gross Unrealized Gains	28	44	130
Securities Held To Maturity, Gross Unrealized Losses		(4)
Securities held to maturity, fair value	4,313	6,892	14,212
Commercial Mortgage-Backed Securities [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	17,560	2,241	1,024
Securities Held To Maturity, Gross Unrealized Gains	179	124	39
Securities Held To Maturity, Gross Unrealized Losses		(37)
Securities held to maturity, fair value	17,739	2,328	1,063
States of the U.S. and Political Subdivisions [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost	150,272	132,162	147,713
Securities Held To Maturity, Gross Unrealized Gains	3,315	1,992	6,099
Securities Held To Maturity, Gross Unrealized Losses	(43)	(2,022)
Securities held to maturity, fair value	153,544	132,132	153,812
Collateralized Debt Obligations [Member]
Investment Securities Held To Maturity [Line Items]
Securities Held To Maturity, Amortized Cost			512
Securities Held To Maturity, Gross Unrealized Losses			(35)
Securities held to maturity, fair value			$ 477

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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

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

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-Publisher FASB

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-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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