Summary of Significant Accounting Policies - Summary of Bank's Actual Capital Amounts and Ratios, and the Regulatory Minimum Ratios (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Regulatory, Minimum, Common equity tier 1 capital	$ 0
Regulatory, Minimum, Tier 1 capital	0
Regulatory, Minimum, Total capital	0
Regulatory, Minimum, Average assets	0
Regulatory, Minimum, Risk-weighted assets	$ 0
Regulatory, Minimum, Leverage ratio	4.00%
Regulatory, Minimum, Common equity tier 1 capital ratio	4.50%
Regulatory, Minimum, Tier 1 capital ratio	6.00%
Regulatory, Minimum, Total capital ratio	8.00%
Regulatory, Well-capitalized, Common equity tier 1 capital	$ 0
Regulatory, Well-capitalized, Tier 1 capital	0
Regulatory, Well-capitalized, Total capital	0
Regulatory, Well-capitalized, Average assets	0
Regulatory, Well-capitalized, Risk-weighted assets	$ 0
Regulatory, Well-capitalized, Leverage ratio	5.00%
Regulatory, Well-capitalized, Common equity tier 1 capital ratio	6.50%
Regulatory, Well-capitalized, Tier 1 capital ratio	8.00%
Regulatory, Well-capitalized, Total capital ratio	10.00%
Common equity tier 1 capital	$ 138,946	$ 137,494
Tier 1 capital	165,249	163,797
Total capital	178,552	176,876
Average assets	1,096,094	1,127,087
Risk-weighted assets	$ 1,010,792	$ 995,145
Leverage ratio	15.10%	14.50%
Common equity tier 1 capital ratio	13.70%	13.80%
Tier 1 capital ratio	16.30%	16.50%
Total capital ratio	17.70%	17.80%