Goodwill and Other Intangible Assets - Schedule of Definite-Lived Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 137,774	$ 129,163
Accumulated Amortization	(59,526)	(53,006)
Net Carrying Amount	78,248	76,157
Core deposit intangible assets
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	136,183	127,957
Accumulated Amortization	(58,161)	(51,971)
Net Carrying Amount	78,022	75,986
Customer relationship intangible asset
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,385	1,143
Accumulated Amortization	(1,305)	(996)
Net Carrying Amount	80	147
Non-compete agreement
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	206	63
Accumulated Amortization	(60)	(39)
Net Carrying Amount	$ 146	$ 24