Employee Benefit Plans (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure
Defined Benefit Plan, Assumed Health Care Cost Trend Rate, Description	0.055
Pension benefits to be amortized from accumulated other comprehensive income (loss) into net periodic cost over the next fiscal year	$ 112,000
Ultimate Health Care Cost Trend Rate	4.00%
Cash and Cash Equivalents	$ 72,194,000	$ 66,146,000	$ 71,115,000	$ 61,454,000
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 1,546,000	1,549,000	1,454,000
Components of net periodic benefit costs:
Time Period of Lower Returns on Equity Securites	10 years
Pension benefits
Defined Benefit Plan Disclosure
Cash and Cash Equivalents	$ 113,000	221,000
Accrued Investment Income Receivable	1,000	12,000
Components of net periodic benefit costs:
Interest cost	451,000	438,000	447,000
Expected return on plan assets	(553,000)	(492,000)	(493,000)
Defined Benefit Plan, Amortization of Gain (Loss)	(102,000)	(95,000)	(117,000)
Settlement of benefit obligation	242,000	0	459,000
Net periodic benefit cost	$ 242,000	$ 41,000	$ 530,000
Discount rate	3.80%	3.90%	3.80%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	7.50%	7.50%	7.50%
Postretirement benefits
Components of net periodic benefit costs:
Interest cost	$ 3,000	$ 4,000	$ 4,000
Expected return on plan assets	0	0	0
Defined Benefit Plan, Amortization of Gain (Loss)	6,000	6,000	5,000
Settlement of benefit obligation	0	0	0
Net periodic benefit cost	$ (3,000)	$ (2,000)	$ (1,000)
Discount rate	3.80%	3.90%	3.50%
Postretirement benefits | Employees hired before January 1, 2003
Defined Benefit Plan Disclosure
Contribution per eligible employee, coverage basis, consecutive period of highest average monthly compensation	5 years
Contribution per eligible employee, coverage basis, lastest service period	10 years
Postretirement benefits | Employees hired on or after January 1, 2003
Defined Benefit Plan Disclosure
Percentage of annual contribution per eligible employee	2.00%
Postretirement Health Coverage [Member] | Employees retired before January 1, 1993
Defined Benefit Plan Disclosure
Maximum coverage per eligible retiree	100.00%
Minimum
Defined Benefit Plan Disclosure
Defined Contribution Plan, Employer Matching Contribution, Percent	0.00%
Defined Contribution Plan, Maximum Annual Contribution Per Employee, Percent	3.00%
Components of net periodic benefit costs:
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	$ 410,000
Minimum | Equity securities
Defined Benefit Plan Disclosure
Actual Plan Asset Allocations	60.00%
Minimum | Investment securities
Defined Benefit Plan Disclosure
Actual Plan Asset Allocations	2000.00%
Minimum | Other Investments [Member]
Defined Benefit Plan Disclosure
Actual Plan Asset Allocations	0.00%
Maximum
Defined Benefit Plan Disclosure
Defined Contribution Plan, Maximum Annual Contribution Per Employee, Percent	5.00%
Components of net periodic benefit costs:
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	$ 440,000
Maximum | Equity securities
Defined Benefit Plan Disclosure
Actual Plan Asset Allocations	75.00%
Maximum | Investment securities
Defined Benefit Plan Disclosure
Actual Plan Asset Allocations	40.00%
Maximum | Other Investments [Member]
Defined Benefit Plan Disclosure
Actual Plan Asset Allocations	0.00%