Intangible Assets - Summary of Intangible Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 32,025	$ 23,585
Accumulated Amortization	(13,243)	(12,503)
Net Carrying Amount	18,782	11,082
Core Deposit Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	25,907	17,447
Accumulated Amortization	(9,098)	(8,451)
Net Carrying Amount	16,809	8,996
Customer Relationship Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	5,699	5,699
Accumulated Amortization	(3,854)	(3,767)
Net Carrying Amount	1,845	1,932
Mortgage Servicing Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	419	439
Accumulated Amortization	(291)	(285)
Net Carrying Amount	$ 128	$ 154