Goodwill and Intangible Assets - Schedule of Estimated Amortization Expense (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets [Line Items]
2018	$ 109
2019	130
2020	114
2021	110
2022	109
Thereafter	687
Net Carrying Amount	1,259	$ 1,279
MSRs [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
2018	31
2019	42
2020	42
2021	42
2022	41
Thereafter	558
Net Carrying Amount	756	743
Core deposit intangibles [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
2018	78
2019	88
2020	72
2021	68
2022	68
Thereafter	129
Net Carrying Amount	$ 503	$ 536