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, 2019	Dec. 31, 2018
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	7.00%
Regulatory, Minimum, Tier 1 capital ratio	8.50%
Regulatory, Minimum, Total capital ratio	10.50%
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	$ 153,580	$ 141,608
Tier 1 capital	179,883	167,911
Total capital	194,436	180,917
Average assets	1,130,642	1,059,461
Risk-weighted assets	$ 1,120,179	$ 993,374
Leverage ratio	15.90%	15.80%
Common equity Tier 1 capital ratio	13.70%	14.30%
Tier 1 capital ratio	16.10%	16.90%
Total capital ratio	17.40%	18.20%