Fair Value Measurements - Balances of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale securities	$ 1,578,526	$ 1,534,065
Derivative assets	66,293	45,898
Derivative liabilities	62,806	46,160
U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	30,004	29,682
U.S. Government-Sponsored Entities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	390,767	337,133
Agency Collateralized Mortgage Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	527,928	573,171
Assets measured at fair value, Available for sale securities	527,928
Non-Agency Collateralized Mortgage Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	1,227	1,431
Assets measured at fair value, Available for sale securities	1,227
Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	4,392	7,880
Assets measured at fair value, Available for sale securities	4,392
States of the U.S. and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	11,272	13,158
Other Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	14,553	16,178
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale securities	224	218
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale securities	1,576,699	1,531,952
Derivative assets	66,293	45,898
Derivative liabilities	62,806	46,160
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale securities	1,603	1,895
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	1,577,263	1,532,718
Assets measured at fair value, Available for sale equity securities	1,263	1,347
Assets measured at fair value, Available for sale securities	1,578,526	1,534,065
Derivative assets	66,293	45,898
Assets measured at fair value, Total	1,644,819	1,579,963
Liabilities measured at fair value, Total	62,806	46,160
Fair Value, Measurements, Recurring [Member] | U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	30,004	29,682
Fair Value, Measurements, Recurring [Member] | U.S. Government-Sponsored Entities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	390,767	337,133
Fair Value, Measurements, Recurring [Member] | Agency Collateralized Mortgage Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	527,928	573,171
Fair Value, Measurements, Recurring [Member] | Non-Agency Collateralized Mortgage Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	1,227	1,431
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	4,392	7,880
Fair Value, Measurements, Recurring [Member] | States of the U.S. and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	11,272	13,158
Fair Value, Measurements, Recurring [Member] | Other Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	14,553	16,178
Fair Value, Measurements, Recurring [Member] | Trading Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	61,361	43,789
Fair Value, Measurements, Recurring [Member] | Not for Trading Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	4,932	2,109
Fair Value, Measurements, Recurring [Member] | Financial Services Industry [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	1,134	1,228
Fair Value, Measurements, Recurring [Member] | Insurance Services Industry [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	129	119
Fair Value, Measurements, Recurring [Member] | Agency Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	597,120	554,085
Fair Value, Measurements, Recurring [Member] | Trading Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	61,766	43,830
Fair Value, Measurements, Recurring [Member] | Not for Trading Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	1,040	2,330
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	224	218
Assets measured at fair value, Available for sale securities	224	218
Assets measured at fair value, Total	224	218
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Financial Services Industry [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	95	99
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Insurance Services Industry [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	129	119
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	1,576,043	1,531,298
Assets measured at fair value, Available for sale equity securities	656	654
Assets measured at fair value, Available for sale securities	1,576,699	1,531,952
Derivative assets	66,293	45,898
Assets measured at fair value, Total	1,642,992	1,577,850
Liabilities measured at fair value, Total	62,806	46,160
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	30,004	29,682
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Government-Sponsored Entities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	390,767	337,133
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Agency Collateralized Mortgage Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	527,928	573,171
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Non-Agency Collateralized Mortgage Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	7	11
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	4,392	7,880
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | States of the U.S. and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	11,272	13,158
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Other Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	14,553	16,178
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Trading Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	61,361	43,789
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Not for Trading Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	4,932	2,109
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Financial Services Industry [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	656	654
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Agency Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	597,120	554,085
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Trading Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	61,766	43,830
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Not for Trading Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	1,040	2,330
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	1,220	1,420
Assets measured at fair value, Available for sale equity securities	383	475
Assets measured at fair value, Available for sale securities	1,603	1,895
Assets measured at fair value, Total	1,603	1,895
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Non-Agency Collateralized Mortgage Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale debt securities	1,220	1,420
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Financial Services Industry [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value, Available for sale equity securities	$ 383	$ 475