Investment Securities - Schedule of Amortized Cost and Fair Values of Investment Securities, with Gross Unrealized Gains and Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Amortized Cost	$ 2,316,690	$ 2,147,276
Securities available for sale, Gross Unrealized Gains	30,149	22,296
Securities available for sale, Gross Unrealized Losses	(4,226)	(10,719)
Securities available for sale, Estimated Fair Value	2,342,613us-gaap_AvailableForSaleSecurities	2,158,853us-gaap_AvailableForSaleSecurities
Securities held to maturity, Amortized Cost	113,442us-gaap_HeldToMaturitySecurities	116,960us-gaap_HeldToMaturitySecurities
Securities held to maturity, Gross Unrealized Gains	3,536	3,392
Securities held to maturity, Gross Unrealized Losses	(607)	(871)
Securities held to maturity, Estimated Fair Value	116,371	119,481
US Treasury Securities [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Amortized Cost	67,196
Securities available for sale, Gross Unrealized Gains	10
Securities available for sale, Estimated Fair Value	67,206
U.S. Government-Sponsored Enterprise Obligations [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Amortized Cost	316,928	317,386
Securities available for sale, Gross Unrealized Gains	2,573	1,700
Securities available for sale, Gross Unrealized Losses	(353)	(3,533)
Securities available for sale, Estimated Fair Value	319,148	315,553
Securities held to maturity, Amortized Cost	10,000	10,000
Securities held to maturity, Gross Unrealized Gains	28	88
Securities held to maturity, Estimated Fair Value	10,028	10,088
Obligations of State and Political Obligations [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Amortized Cost	78,437	86,513
Securities available for sale, Gross Unrealized Gains	3,600	3,679
Securities available for sale, Gross Unrealized Losses	(2)	(2)
Securities available for sale, Estimated Fair Value	82,035	90,190
Securities held to maturity, Amortized Cost	75,458	77,597
Securities held to maturity, Gross Unrealized Gains	3,353	3,153
Securities held to maturity, Gross Unrealized Losses	(52)	(145)
Securities held to maturity, Estimated Fair Value	78,759	80,605
Mortgage-Backed Securities [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Amortized Cost	1,821,982	1,741,917
Securities available for sale, Gross Unrealized Gains	23,788	16,882
Securities available for sale, Gross Unrealized Losses	(3,857)	(7,184)
Securities available for sale, Estimated Fair Value	1,841,913	1,751,615
Securities held to maturity, Amortized Cost	27,984	29,363
Securities held to maturity, Gross Unrealized Gains	155	151
Securities held to maturity, Gross Unrealized Losses	(555)	(726)
Securities held to maturity, Estimated Fair Value	27,584	28,788
Other Securities [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Securities available for sale, Amortized Cost	32,147	1,460
Securities available for sale, Gross Unrealized Gains	178	35
Securities available for sale, Gross Unrealized Losses	(14)
Securities available for sale, Estimated Fair Value	$ 32,311	$ 1,495

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

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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

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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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