Regulatory Matters - Additional Information (Detail) - USD ($) $ in Millions		12 Months Ended
	Dec. 06, 2017	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Dec. 31, 2016	Jul. 31, 2013
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Capital to Risk Weighted Assets			11.11%		9.42%
Total Capital to Risk Weighted Assets			14.00%		12.44%
Proceeds from public offering of common stock	$ 70.5
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets			6.00%		6.00%
Capital Required for Capital Adequacy to Risk Weighted Assets			8.00%		8.00%
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets			8.00%		8.00%
Capital Required to be Well Capitalized to Risk Weighted Assets			10.00%		10.00%
Amount available for dividend distribution without prior approval from Regulatory Agency			$ 36.5
Percentage of Bank's capital and surplus of which extensions of credit to a single affiliate are limited			10.00%
Percentage of Bank's capital and surplus of which extensions of credit to all affiliates are limited			20.00%
Basel III New Requirements [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 capital minimum buffer percent						2.50%
Capital conservation buffer requirement phase in period in years			4 years
Minimum Capital Requirements Effective Date			Jan. 01, 2015
Basel III New Requirements [Member] | Scenario, Forecast [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Capital Conservation Buffer Above Minimum Risk-based Capital Requirements		1.875%
Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Capital to Risk Weighted Assets			11.07%		11.36%
Total Capital to Risk Weighted Assets			11.62%		11.85%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets			6.00%		6.00%
Capital Required for Capital Adequacy to Risk Weighted Assets			8.00%		8.00%
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets			8.00%		8.00%
Capital Required to be Well Capitalized to Risk Weighted Assets			10.00%		10.00%
Minimum [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Excess Tier One Risk Based Capital to Risk Weighted Assets			1.25%
Minimum [Member] | Scenario, Forecast [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Capital to Risk Weighted Assets		8.50%
Total Capital to Risk Weighted Assets				10.50%
Minimum [Member] | Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Excess Tier One Risk Based Capital to Risk Weighted Assets			1.25%