Derivative Instruments - Amounts and Locations of Activity Related to Back-to-Back Interest Rate Swaps within Corporation's Consolidated Balance Sheet (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
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	$ 11,772,000	$ 11,848,000
Weighted Average Maturity (in years)	7 years 9 months 18 days	8 years
Weighted Average Fixed Rate	4.52%	4.51%
Weighted Average Variable Rate, Description	1 month LIBOR + 2.37%	1 month LIBOR + 2.37%
Weighted Average Variable Rate	2.37%	2.37%
Fair Value	$ (146,000)	$ 149,000
Customer Interest Rate Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Notional Amount	$ 11,772,000	$ 11,848,000
Weighted Average Maturity (in years)	7 years 9 months 18 days	8 years
Weighted Average Fixed Rate	4.52%	4.51%
Weighted Average Variable Rate, Description	1 month LIBOR + 2.37%	1 month LIBOR + 2.37%
Weighted Average Variable Rate	2.37%	2.37%
Fair Value	$ 146,000	$ (149,000)