Derivatives and Hedging Activities - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) Derivative	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Derivative	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Derivative
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregate notional amount	$ 305,600,000		$ 305,600,000		$ 298,200,000
Net gain (loss) on change in fair value	40,000	$ (108,000)	753,000	$ (180,000)
Collateral posted with market value on liability positions with credit risk-related contingent features	$ 2,300,000		$ 2,300,000
Interest Rate Swaps [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of interest rate swaps | Derivative	42		42		39
Net gain (loss) on change in fair value			$ 200,000	(300,000)
Income (loss) on derivative instrument not designated hedges			$ 500,000	$ 1,100,000