Derivatives - Summary Information Regarding Derivatives (Detail) - USD ($)			12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
3rd Party interest rate swaps
Derivative [Line Items]
Derivatives, asset notional amount	$ 110,076,000		$ 110,076,000
Derivatives liability, notional amount	$ 82,760,000		$ 82,760,000
Notional amount		$ (129,252,000)		$ (129,252,000)
Derivatives, asset average maturity (years)	8 years 9 months 18 days
Derivative, liability average maturity (years)	11 years 6 months
Average Maturity (Years)		10 years 10 months 24 days
Weighted average rate fixed, other assets	3.87%		3.87%
Weighted average rate fixed, other liabilities	4.74%		4.74%
Weighted Average Rate Fixed		4.03%		4.03%
Weighted average variable rate description, other assets			1 Mo. LIBOR + 2.11%
Weighted average variable rate, other liabilities			1 Mo. LIBOR + 2.21%
Weighted Average Variable Rate				1 Mo. LIBOR + 2.10%
Derivatives liability, asset fair value	$ (3,634,000)		$ (3,634,000)
Derivative liability, liability fair value	$ (1,831,000)		$ (1,831,000)
Fair Value		$ 2,345,000		$ 2,345,000
Derivative asset, average basis spread on variable rate	2.00%		2.00%
Derivative liability, average basis spread on variable rate	2.00%		2.00%
Derivatives, weighted average variable rate		2.10%		2.10%
Customer interest rate swaps
Derivative [Line Items]
Derivatives, asset notional amount	$ 82,760,000		$ 82,760,000
Derivatives liability, notional amount	$ 110,076,000		$ 110,076,000
Notional amount		$ (129,252,000)		$ (129,252,000)
Derivatives, asset average maturity (years)	11 years 6 months
Derivative, liability average maturity (years)	8 years 9 months 18 days
Average Maturity (Years)		10 years 10 months 24 days
Weighted average rate fixed, other assets	4.74%		4.74%
Weighted average rate fixed, other liabilities	3.87%		3.87%
Weighted Average Rate Fixed		4.03%		4.03%
Weighted average variable rate description, other assets			1 Mo. LIBOR + 2.21%
Weighted average variable rate, other liabilities			1 Mo. LIBOR + 2.11%
Weighted Average Variable Rate				1 Mo. LIBOR + 2.10%
Derivatives liability, asset fair value	$ (1,831,000)		$ (1,831,000)
Derivative liability, liability fair value	$ (3,634,000)		$ (3,634,000)
Fair Value		$ (2,345,000)		$ (2,345,000)
Derivative asset, average basis spread on variable rate	2.00%		2.00%
Derivative liability, average basis spread on variable rate	2.00%		2.00%
Derivatives, weighted average variable rate		2.10%		2.10%
Interest rate swap (cash flow hedge)
Derivative [Line Items]
Derivatives, asset notional amount	$ 30,000,000		$ 30,000,000
Notional amount		$ (30,000,000)		$ (30,000,000)
Derivatives, asset average maturity (years)	3 years 6 months
Average Maturity (Years)		4 years 6 months
Weighted average rate fixed, other assets	1.10%		1.10%
Weighted Average Rate Fixed		1.10%		1.10%
Weighted average variable rate description, other assets			3 Mo. LIBOR
Weighted Average Variable Rate				3 Mo. LIBOR
Derivatives liability, asset fair value	$ (1,090,000)		$ (1,090,000)
Fair Value		$ 1,033,000		$ 1,033,000