Fair Value Measurements - Balances of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	$ 1,532,718	$ 1,139,524
Assets measured at fair value, Available for sale equity securities	1,347	2,126
Assets measured at fair value, Available for sale securities	1,534,065	1,141,650	1,172,683
Derivative financial instruments	45,898	33,317
Assets measured at fair value, Total	1,579,963	1,174,967
Derivative financial instruments	46,160	43,369
Liabilities measured at fair value, Total	46,160	43,369
U.S. Treasury [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	29,682
U.S. Government-Sponsored Entities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	337,133	330,985	354,457
States of the U.S. and Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	13,158	17,002	24,824
Other Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	16,178	16,100	21,513
Agency Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	554,085	250,881
Agency Collateralized Mortgage Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	573,171	491,199
Non-Agency Collateralized Mortgage Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	1,431	1,762
Commercial Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	7,880
Financial Services Industry [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	1,228	2,061
Insurance Services Industry [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	119	65
Collateralized Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities		31,595
Trading Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative financial instruments	43,789	33,317
Not for Trading Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative financial instruments	2,109	0
Trading Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative financial instruments	43,830	33,236
Not for Trading Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative financial instruments	2,330	10,133
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	218	649
Assets measured at fair value, Available for sale securities	218	649
Assets measured at fair value, Total	218	649
Level 1 [Member] | Financial Services Industry [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	99	584
Level 1 [Member] | Insurance Services Industry [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	119	65
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	1,531,298	1,106,185
Assets measured at fair value, Available for sale equity securities	654	1,067
Assets measured at fair value, Available for sale securities	1,531,952	1,107,252
Derivative financial instruments	45,898	33,317
Assets measured at fair value, Total	1,577,850	1,140,569
Derivative financial instruments	46,160	43,369
Liabilities measured at fair value, Total	46,160	43,369
Level 2 [Member] | U.S. Treasury [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	29,682
Level 2 [Member] | U.S. Government-Sponsored Entities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	337,133	330,985
Level 2 [Member] | States of the U.S. and Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	13,158	17,002
Level 2 [Member] | Other Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	16,178	16,100
Level 2 [Member] | Agency Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	554,085	250,881
Level 2 [Member] | Agency Collateralized Mortgage Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	573,171	491,199
Level 2 [Member] | Non-Agency Collateralized Mortgage Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	11	18
Level 2 [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	7,880
Level 2 [Member] | Financial Services Industry [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	654	1,067
Level 2 [Member] | Trading Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative financial instruments	43,789	33,317
Level 2 [Member] | Not for Trading Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative financial instruments	2,109
Level 2 [Member] | Trading Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative financial instruments	43,830	33,236
Level 2 [Member] | Not for Trading Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative financial instruments	2,330	10,133
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	1,420	33,339
Assets measured at fair value, Available for sale equity securities	475	410
Assets measured at fair value, Available for sale securities	1,895	33,749
Assets measured at fair value, Total	1,895	33,749
Level 3 [Member] | Non-Agency Collateralized Mortgage Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	1,420	1,744
Level 3 [Member] | Financial Services Industry [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	475	410
Level 3 [Member] | Collateralized Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities		$ 31,595