Derivatives - Summary Information Regarding Derivatives (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
3rd Party interest rate swaps
Derivative [Line Items]
Other Assets, Notional Amount	$ 175,920,000	$ 110,076,000
Other Assets, Average Maturity (Years)	9 years	8 years 9 months 18 days
Other Assets, Wighted Average Fixed Rate	4.20%	3.87%
Other Assets, Weighted Average Variable Rate	1 Mo. LIBOR + 2.17%	1 Mo. LIBOR + 2.11%
Other Assets, Fair Value	$ 7,016,000	$ 3,634,000
Other Assets, Average Basis Spread On Variable Rate	2.17%	2.11%
Other Liabilities, Notional Amount	$ 43,918,000	$ 82,760,000
Other Liabilities, Average Maturity (Years)	13 years 10 months 24 days	11 years 6 months
Other Liabilities, Weighted Average Fixed Rate	5.27%	4.74%
Other Liabilities, Weighted Average Variable Rate	1 Mo. LIBOR + 2.32%	1 Mo. LIBOR + 2.21%
Other Liabilities, Fair Value	$ (796,000)	$ (1,831,000)
Other Liabilities, Average Basis Spread On Variable Rate	2.32%	2.21%
Customer interest rate swaps
Derivative [Line Items]
Other Assets, Notional Amount	$ 43,918,000	$ 82,760,000
Other Assets, Average Maturity (Years)	13 years 10 months 24 days	11 years 6 months
Other Assets, Wighted Average Fixed Rate	5.27%	4.74%
Other Assets, Weighted Average Variable Rate	1 Mo. LIBOR + 2.32%	1 Mo. LIBOR + 2.21%
Other Assets, Fair Value	$ 796,000	$ 1,831,000
Other Assets, Average Basis Spread On Variable Rate	2.32%	2.21%
Other Liabilities, Notional Amount	$ 175,920,000	$ 110,076,000
Other Liabilities, Average Maturity (Years)	9 years	8 years 9 months 18 days
Other Liabilities, Weighted Average Fixed Rate	4.20%	3.87%
Other Liabilities, Weighted Average Variable Rate	1 Mo. LIBOR + 2.17%	1 Mo. LIBOR + 2.11%
Other Liabilities, Fair Value	$ (7,016,000)	$ (3,634,000)
Other Liabilities, Average Basis Spread On Variable Rate	2.17%	2.11%
Interest rate swap (cash flow hedge)
Derivative [Line Items]
Other Assets, Notional Amount	$ 30,000,000	$ 30,000,000
Other Assets, Average Maturity (Years)	3 years 3 months	3 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	$ 1,449,000	$ 1,090,000