Derivatives - Summary Information Regarding Derivatives (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
3rd Party Interest Rate Swaps [Member]
Derivative [Line Items]
Derivatives, Asset Notional Amount	$ 87,511,000
Derivatives, Liability Notional Amount		$ 129,252,000
Derivatives, Average Maturity (Years)	9 years 2 months 12 days	10 years 10 months 25 days
Derivatives, Weighted Average Rate Fixed	3.78%	4.03%
Derivatives, Weighted Average Variable Rate, description	1 Mo. LIBOR + 2.12	1 Mo. LIBOR + 2.10
Derivatives, Weighted Average Variable Rate	2.12%	2.10%
Derivatives, Fair Value	$ 2,831,000	$ 2,345,000
Customer Interest Rate Swaps [Member]
Derivative [Line Items]
Derivatives, Asset Notional Amount	$ 98,223,000	$ 129,252,000
Derivatives, Average Maturity (Years)	11 years 1 month 6 days	10 years 10 months 25 days
Derivatives, Weighted Average Rate Fixed	4.64%	4.03%
Derivatives, Weighted Average Variable Rate, description	1 Mo. LIBOR + 2.19	1 Mo. LIBOR + 2.10
Derivatives, Weighted Average Variable Rate	2.19%	2.10%
Derivatives, Fair Value	$ 2,549,000	$ (2,345,000)
Interest Rate Swap Cash Flow Hedge [Member]
Derivative [Line Items]
Derivatives, Asset Notional Amount	$ 30,000,000	$ 30,000,000
Derivatives, Average Maturity (Years)	3 years 9 months 18 days	4 years 6 months
Derivatives, Weighted Average Rate Fixed	1.10%	1.10%
Derivatives, Weighted Average Variable Rate, description	3 Mo. LIBOR	3 Mo. LIBOR
Derivatives, Fair Value	$ 871,000	$ 1,033,000
Customer Interest Rate Swaps [Member]
Derivative [Line Items]
Derivatives, Liability Notional Amount	$ 87,511,000
Derivatives, Average Maturity (Years)	9 years 2 months 12 days
Derivatives, Weighted Average Rate Fixed	3.78%
Derivatives, Weighted Average Variable Rate, description	1 Mo. LIBOR + 2.12
Derivatives, Weighted Average Variable Rate	2.12%
Derivatives, Fair Value	$ (2,831,000)
3rd Party Interest Rate Swaps [Member]
Derivative [Line Items]
Derivatives, Liability Notional Amount	$ 98,223,000
Derivatives, Average Maturity (Years)	11 years 1 month 6 days
Derivatives, Weighted Average Rate Fixed	4.64%
Derivatives, Weighted Average Variable Rate, description	1 Mo. LIBOR + 2.19
Derivatives, Weighted Average Variable Rate	2.19%
Derivatives, Fair Value	$ (2,549,000)