Derivative Instruments - Amounts and Locations of Activity Related to Back-to-Back Interest Rate Swaps within Corporation's Consolidated Balance Sheet (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
3rd Party Interest Rate Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Notional Amount	$ 14,559,000	$ 14,814,000
Weighted Average Maturity (in years)	8 years 3 months 19 days	9 years
Weighted Average Fixed Rate	4.43%	4.43%
Weighted Average Variable Rate, Description	1 month LIBOR + 2.35%	1 month LIBOR + 2.35%
Weighted Average Variable Rate	2.35%	2.35%
Fair Value	$ 233,000	$ 211,000
Customer Interest Rate Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Notional Amount	$ 14,559,000	$ 14,814,000
Weighted Average Maturity (in years)	8 years 3 months 19 days	9 years
Weighted Average Fixed Rate	4.43%	4.43%
Weighted Average Variable Rate, Description	1 month LIBOR + 2.35%	1 month LIBOR + 2.35%
Weighted Average Variable Rate	2.35%	2.35%
Fair Value	$ 233,000	$ 211,000