Intangible Assets and Goodwill - Summary of Intangible Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 23,585	$ 23,619
Accumulated Amortization	(12,503)	(10,289)
Net Carrying Amount	11,082	13,330
Core Deposit Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	17,447	17,447
Accumulated Amortization	(8,451)	(6,611)
Net Carrying Amount	8,996	10,836
Customer Relationship Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	5,699	5,699
Accumulated Amortization	(3,767)	(3,419)
Net Carrying Amount	1,932	2,280
Mortgage Servicing Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	439	473
Accumulated Amortization	(285)	(259)
Net Carrying Amount	$ 154	$ 214