Goodwill and Intangible Assets - Schedule of Acquired Intangible Assets, Other than Goodwill (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 8,368	$ 8,339
Accumulated Amortization	7,109	7,060
Net Carrying Amount	1,259	1,279
MSRs [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,094	1,065
Accumulated Amortization	338	322
Net Carrying Amount	756	743
Core deposit intangibles [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,274	7,274
Accumulated Amortization	6,771	6,738
Net Carrying Amount	$ 503	$ 536