Post-Retirement Benefits (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Post-Retirement Benefits
Number of post-retirement health and life insurance benefit plans | item	2
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Net periodic pension expense	$ (351,000)	$ (205,000)	$ (152,000)
Total amount recognized in other comprehensive income	589,000	1,202,000	2,229,000
Effects of one-percentage point change in assumed health care cost trend rates
One-Percentage Point Increase, Effect on aggregate service and interest cost	6,000
One-Percentage Point Decrease, Effect on aggregate service and interest cost	(6,000)
One-Percentage Point Increase, Effect on postretirement benefit obligation	166
One-Percentage Point Decrease, Effect on postretirement benefit obligation	(150)
Other Comprehensive Income
Change in benefit obligation:
Interest cost	1,124,000	1,132,000	1,017,000
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	1,124,000	1,132,000	1,017,000
Recognized net actuarial loss	751,000	816,000	898,000
Net periodic pension expense	(211,000)	(71,000)	(152,000)
Net (gain) loss	491,000	655,000	2,315,000
Amortization of loss	(751,000)	(816,000)	(898,000)
Total amount recognized in other comprehensive income	(260,000)	(161,000)	1,417,000
Total recognized in net periodic benefit cost and other comprehensive income	(471,000)	(232,000)	1,265,000
Retiree Medical Plan
Change in benefit obligation:
Benefit obligation at beginning of year	393,000	422,000	436,000
Interest cost	13,000	13,000	12,000
Actuarial (gain) loss	(29,000)	1,000	19,000
Benefits paid	(40,000)	(43,000)	(45,000)
Benefit obligation at end of year	337,000	393,000	422,000
Change in plan assets:
Employer contribution	40,000	43,000	45,000
Benefits paid	(40,000)	(43,000)	(45,000)
Funded status	$ (337,000)	$ (393,000)	$ (422,000)
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)	3.20%	3.50%	3.30%
Weighted-average assumptions used to determine net periodic pension cost
Discount rate (as a percent)	3.50%	3.30%	3.00%
Assumed health care cost trend rates
Health care cost trend rate assumed for next fiscal year (as a percent)	5.00%	5.00%	5.00%
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	$ 13,000	$ 13,000	$ 12,000
Recognized net actuarial loss	10,000	10,000	9,000
Net periodic pension expense	23,000	23,000	21,000
Net (gain) loss	(29,000)	1,000	19,000
Amortization of loss	(10,000)	(10,000)	(9,000)
Total amount recognized in other comprehensive income	(39,000)	(9,000)	10,000
Total recognized in net periodic benefit cost and other comprehensive income	(16,000)	14,000	31,000
Estimated net loss for the retiree medical plan that will be amortized from other comprehensive income into periodic benefit cost over the next fiscal year
Estimated net loss for the plan that will be amortized from other comprehensive income into periodic benefit cost over the next fiscal year	6,000,000
Effects of one-percentage point change in assumed health care cost trend rates
One-Percentage Point Increase, Effect on aggregate service and interest cost	1,000
One-Percentage Point Decrease, Effect on aggregate service and interest cost	(1,000)
One-Percentage Point Increase, Effect on postretirement benefit obligation	19,000
One-Percentage Point Decrease, Effect on postretirement benefit obligation	(17,000)
Estimated future benefit payments (including expected future service as appropriate)
2018	38,000
2019	37,000
2020	36,000
2021	34,000
2022	33,000
2023-2027	131,000
Estimated future benefit payments	309,000
Expected contributions in the next fiscal year
Amount of expected contributions in the next fiscal year	38,000
Retiree Welfare Plan
Change in benefit obligation:
Benefit obligation at beginning of year	2,447,000	2,092,000	2,331,000
Interest cost	81,000	74,000	77,000
Actuarial (gain) loss	126,000	546,000	(105,000)
less: federal subsidy on benefits paid	12,000	15,000	22,000
Benefit obligation at end of year	2,392,000	2,447,000	2,092,000
Change in plan assets:
Employer contribution	274,000	280,000	233,000
Benefits paid	(274,000)	(280,000)	(233,000)
Funded status	$ (2,392,000)	$ (2,447,000)	$ (2,092,000)
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)	3.20%	3.50%	3.70%
Weighted-average assumptions used to determine net periodic pension cost
Discount rate (as a percent)	3.50%	3.70%	3.45%
Assumed health care cost trend rates
Health care cost trend rate assumed for next fiscal year (as a percent)	5.00%	5.00%	6.25%
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	$ 81,000	$ 74,000	$ 77,000
Estimated net loss for the retiree medical plan that will be amortized from other comprehensive income into periodic benefit cost over the next fiscal year
Estimated net loss for the plan that will be amortized from other comprehensive income into periodic benefit cost over the next fiscal year	154,000
Estimated future benefit payments (including expected future service as appropriate)
2018	241,000
2019	236,000
2020	230,000
2021	222,000
2022	213,000
2023-2027	894,000
Estimated future benefit payments	2,036,000
Expected contributions in the next fiscal year
Amount of expected contributions in the next fiscal year	241,000
Retiree Welfare Plan | Other Comprehensive Income
Change in benefit obligation:
Interest cost	81,000	74,000	77,000
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	81,000	74,000	77,000
Recognized net actuarial loss	147,000	95,000	115,000
Net periodic pension expense	228,000	169,000	192,000
Net (gain) loss	126,000	546,000	(106,000)
Amortization of loss	(147,000)	(95,000)	(115,000)
Total amount recognized in other comprehensive income	(21,000)	451,000	(221,000)
Total recognized in net periodic benefit cost and other comprehensive income	$ 207,000	$ 620,000	$ (29,000)