Goodwill and Other Intangible Assets - Schedule of Definite-Lived Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 159,170	$ 75,412
Accumulated Amortization	(48,360)	(53,251)
Net Carrying Amount	110,810	22,161
Core deposit intangible assets
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	157,964	74,001
Accumulated Amortization	(47,355)	(52,165)
Net Carrying Amount	110,609	21,836
Customer relationship intangible asset
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,143	1,348
Accumulated Amortization	(970)	(1,064)
Net Carrying Amount	173	284
Non-compete agreement
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	63	63
Accumulated Amortization	(35)	(22)
Net Carrying Amount	$ 28	$ 41