Derivative Instruments and Hedging Activities - Derivatives Designated as Hedging Instruments (Detail) - Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	$ 19,419		$ 19,419		$ 19,993
Derivative Assets	0		0		0
Derivative Liabilities	202		202		254
Net loss	(41)	$ (76)	(143)	$ (237)
Gain (loss) recognized in other comprehensive income (loss)			(113)		(141)
Interest Rate Swap [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	18,021		18,021		18,566
Gain (loss) recognized in other comprehensive income (loss)			(113)		(141)
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net loss	41	76	148	237
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | Other Assets [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Assets	0		0		0
Interest Rate Swap [Member] | Cash Flow Hedging [Member] | Other Liabilities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liabilities	174		174		217
Interest Rate Swap [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	1,398		1,398		1,427
Interest Rate Swap [Member] | Fair Value Hedging [Member] | Other Noninterest Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net loss	0	$ 0	5	$ 0
Interest Rate Swap [Member] | Fair Value Hedging [Member] | Other Assets [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Assets	0		0		0
Interest Rate Swap [Member] | Fair Value Hedging [Member] | Other Liabilities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liabilities	$ 28		$ 28		$ 37