Goodwill and Other Intangible Assets - Schedule of Definite-Lived Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 137,774	$ 137,774
Accumulated Amortization	(78,721)	(64,608)
Net Carrying Amount	59,053	73,166
Core deposit intangible assets
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	136,183	136,183
Accumulated Amortization	(77,256)	(63,213)
Net Carrying Amount	58,927	72,970
Customer relationship intangible asset
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,385	1,385
Accumulated Amortization	(1,363)	(1,323)
Net Carrying Amount	22	62
Non-compete agreement
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	206	206
Accumulated Amortization	(102)	(72)
Net Carrying Amount	$ 104	$ 134