Goodwill and Other Intangible Assets - Schedule of Acquired Intangible Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 13,406	$ 10,352
Accumulated Amortization	9,412	8,991
Core deposit intangibles [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	11,184	8,952
Accumulated Amortization	9,250	8,947
Customer list intangible [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	2,222	1,400
Accumulated Amortization	$ 162	$ 44