Goodwill and Intangible Assets - Schedule of Acquired Intangible Assets, Other than Goodwill (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 8,408	$ 8,339
Accumulated Amortization	7,161	7,060
Net Carrying Amount	1,247	1,279
MSRs [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,134	1,065
Accumulated Amortization	364	322
Net Carrying Amount	770	743
Core deposit intangibles [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,274	7,274
Accumulated Amortization	6,797	6,738
Net Carrying Amount	$ 477	$ 536