Derivatives - Summary Information Regarding Derivatives (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
3rd Party interest rate swaps
Derivative [Line Items]
Other Assets, Notional Amount	$ 88,842	$ 153,909
Other Assets, Average Maturity (Years)	7 years 6 months	8 years 3 months 18 days
Other Assets, Wighted Average Fixed Rate	3.80%	4.10%
Other Assets, Weighted Average Variable Rate	1 Mo. LIBOR + 2.13%	1 Mo. LIBOR + 2.13%
Other Assets, Fair Value	$ 2,939	$ 5,329
Other Assets, Average Basis Spread On Variable Rate	2.13%	2.13%
Other Liabilities, Notional Amount	$ 253,922	$ 164,427
Other Liabilities, Average Maturity (Years)	10 years 10 months 2 days	12 years
Other Liabilities, Weighted Average Fixed Rate	4.83%	5.04%
Other Liabilities, Weighted Average Variable Rate	1 Mo. LIBOR + 2.04%	1 Mo. LIBOR + 2.05%
Other Liabilities, Fair Value	$ (11,411)	$ (5,707)
Other Liabilities, Average Basis Spread On Variable Rate	2.04%	2.05%
Customer interest rate swaps
Derivative [Line Items]
Other Assets, Notional Amount	$ 253,922	$ 164,427
Other Assets, Average Maturity (Years)	10 years 10 months 2 days	12 years
Other Assets, Wighted Average Fixed Rate	4.83%	5.04%
Other Assets, Weighted Average Variable Rate	1 Mo. LIBOR + 2.04%	1 Mo. LIBOR + 2.05%
Other Assets, Fair Value	$ 11,411	$ 5,707
Other Assets, Average Basis Spread On Variable Rate	2.04%	2.05%
Other Liabilities, Notional Amount	$ 88,842	$ 153,909
Other Liabilities, Average Maturity (Years)	7 years 6 months	8 years 3 months 18 days
Other Liabilities, Weighted Average Fixed Rate	3.80%	4.10%
Other Liabilities, Weighted Average Variable Rate	1 Mo. LIBOR + 2.13%	1 Mo. LIBOR + 2.13%
Other Liabilities, Fair Value	$ (2,939)	$ (5,329)
Other Liabilities, Average Basis Spread On Variable Rate	2.13%	2.13%
Interest rate swap (cash flow hedge)
Derivative [Line Items]
Other Assets, Notional Amount	$ 30,000	$ 30,000
Other Assets, Average Maturity (Years)	2 years 3 months	2 years 6 months
Other Assets, Wighted Average Fixed Rate	1.10%	1.10%
Other Assets, Weighted Average Variable Rate	3 Mo. LIBOR	3 Mo. LIBOR
Other Assets, Fair Value	$ 828	$ 1,099