Revenue Recognition, Noninterest Income by Major Source (Parenthetical) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Balance	$ 56,666,000	$ 55,267,000	$ 56,142,000	$ 54,451,000
Other service charges, commissions and other income	91,000	133,000
Income within the scope of ASC 606
Disaggregation Of Revenue [Line Items]
Other service charges, commissions and other income	$ 68,000	$ 66,000