Summary of Significant Accounting Policies - Summary of Bank's Actual Capital Amounts and Ratios, and the Regulatory Minimum Ratios (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 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	$ 143,409	$ 141,608
Tier 1 capital	169,712	167,911
Total capital	182,858	180,917
Average assets	1,025,114	1,059,461
Risk-weighted assets	$ 1,005,656	$ 993,374
Leverage ratio	16.60%	15.80%
Common equity Tier 1 capital ratio	14.30%	14.30%
Tier 1 capital ratio	16.90%	16.90%
Total capital ratio	18.20%	18.20%