Interest Rate Swaps - Amounts and Locations of Activity Related to Back-to-Back Interest Rate Swaps within Corporation's Consolidated Balance Sheet (Detail) - USD ($)	12 Months Ended
	Dec. 31, 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	$ 11,848,000	$ 14,814,000
Average Maturity (in years)	8 years	9 years 8 months 12 days
Weighted Average Fixed Rate	4.51%	4.43%
Weighted Average Variable Rate, description	1 month LIBOR + 2.37%	1 month LIBOR + 2.35%
Weighted Average Variable Rate	2.37%	2.35%
Fair Value	$ 149,000	$ 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	$ (11,848,000)	$ (14,814,000)
Average Maturity (in years)	8 years	9 years 8 months 12 days
Weighted Average Fixed Rate	4.51%	4.43%
Weighted Average Variable Rate, description	1 month LIBOR + 2.37%	1 month LIBOR + 2.35%
Weighted Average Variable Rate	2.37%	2.35%
Fair Value	$ (149,000)	$ (211,000)