Fair Value (Additional Information) (Details) - USD ($)		3 Months Ended		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan held for sale, at fair value		$ 6,628,000			$ 6,719,000
Contractual principal balance of loan held for sale		6,478,000			6,576,000
Difference in amount of loan held for sale		150,000			143,000
Impaired loans, carrying amount	[1]	8,093,000			9,293,000
Valuation allowance for loans losses		1,217,000			2,234,000
Other real estate, carrying value		0			0
Cost of real estate sales		0	$ 0
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities		737,957,000			740,994,000
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities		5,524,000			6,002,000
Obligations of State and Political Subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total gains/loss included in earnings attributable to other changes in fair value		(18,000)	17,000
Obligations of State and Political Subdivisions | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities		272,753,000			273,309,000
Total gains/loss included in earnings attributable to other changes in fair value		(18,000)	17,000
Obligations of State and Political Subdivisions | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities		5,171,000			5,649,000
Equity Securities | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities		353,000			353,000
Total gains/loss included in earnings attributable to other changes in fair value		0	0
Equity Securities | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities		353,000			353,000
Commercial Real Estate Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Impaired loans, carrying amount		7,274,000			9,020,000
Valuation allowance for loans losses		1,181,000			$ 2,228,000
Increase (decrease) to provision for loan losses		$ 1,047,000	$ 278,000	$ 1,973,000

[1] Unpaid Principal Balance is the remaining contractual principal payments gross of partial charge-offs and discounts.