Derivatives - Summary Information Regarding Derivatives (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
3rd Party interest rate swaps
Derivative [Line Items]
Other Assets, Notional Amount	$ 203,267	$ 110,076
Other Assets, Average Maturity (Years)	9 years 2 months 23 days	8 years 9 months 18 days
Other Assets, Wighted Average Fixed Rate	4.26%	3.87%
Other Assets, Weighted Average Variable Rate	1 Mo. LIBOR + 2.11%	1 Mo. LIBOR + 2.11%
Other Assets, Fair Value	$ 10,575	$ 3,634
Other Assets, Average Basis Spread On Variable Rate	2.15%	2.11%
Other Liabilities, Notional Amount	$ 81,180	$ 82,760
Other Liabilities, Average Maturity (Years)	12 years 10 months 2 days	11 years 6 months
Other Liabilities, Weighted Average Fixed Rate	5.25%	4.74%
Other Liabilities, Weighted Average Variable Rate	1 Mo. LIBOR + 2.14%	1 Mo. LIBOR + 2.21%
Other Liabilities, Fair Value	$ (815)	$ (1,831)
Other Liabilities, Average Basis Spread On Variable Rate	2.12%	2.21%
Customer interest rate swaps
Derivative [Line Items]
Other Assets, Notional Amount	$ 81,180	$ 82,760
Other Assets, Average Maturity (Years)	12 years 10 months 2 days	11 years 6 months
Other Assets, Wighted Average Fixed Rate	5.25%	4.74%
Other Assets, Weighted Average Variable Rate	1 Mo. LIBOR + 2.14%	1 Mo. LIBOR + 2.21%
Other Assets, Fair Value	$ 815	$ 1,831
Other Assets, Average Basis Spread On Variable Rate	2.12%	2.21%
Other Liabilities, Notional Amount	$ 203,267	$ 110,076
Other Liabilities, Average Maturity (Years)	9 years 2 months 23 days	8 years 9 months 18 days
Other Liabilities, Weighted Average Fixed Rate	4.26%	3.87%
Other Liabilities, Weighted Average Variable Rate	1 Mo. LIBOR + 2.11%	1 Mo. LIBOR + 2.11%
Other Liabilities, Fair Value	$ (10,575)	$ (3,634)
Other Liabilities, Average Basis Spread On Variable Rate	2.15%	2.11%
Interest rate swap (cash flow hedge)
Derivative [Line Items]
Other Assets, Notional Amount	$ 30,000	$ 30,000
Other Assets, Average Maturity (Years)	2 years 9 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	$ 1,524	$ 1,090