Goodwill and Other Intangible Assets - Schedule of Definite-Lived Intangible Assets at Carrying Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Acquired Finite-Lived Intangible Assets
Gross carrying amount	$ 79,594	$ 57,665
Accumulated amortization	(43,305)	(37,304)
Net carrying amount	36,289	20,361
Core Deposit Intangibles
Acquired Finite-Lived Intangible Assets
Gross carrying amount	77,941	55,949
Accumulated amortization	(42,195)	(36,354)
Net carrying amount	35,746	19,595
Customer Relationships Intangible Asset
Acquired Finite-Lived Intangible Assets
Gross carrying amount	1,348	1,348
Accumulated amortization	(946)	(822)
Net carrying amount	402	526
Non-Compete Agreements
Acquired Finite-Lived Intangible Assets
Gross carrying amount	100	163
Accumulated amortization	(67)	(82)
Net carrying amount	33	81
Other Intangible Assets
Acquired Finite-Lived Intangible Assets
Gross carrying amount	205	205
Accumulated amortization	(97)	(46)
Net carrying amount	$ 108	$ 159