Post-Retirement Benefits (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 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
Amortization of loss	$ (1,021,000)	$ (761,000)	$ (482,000)
Total amount recognized in other comprehensive income	2,229,000	3,727,000	(5,804,000)
Effects of one-percentage point change in assumed health care cost trend rates
One-Percentage Point Increase, Effect on aggregate service and interest cost	5,000
One-Percentage Point Decrease, Effect on aggregate service and interest cost	(4,000)
Other Comprehensive Income (Loss) [Member]
Change in benefit obligation:
Interest cost	1,017,000	1,110,000	997,000
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	1,017,000	1,110,000	997,000
Recognized net actuarial loss	898,000	659,000	1,206,000
Net periodic pension expense	(152,000)	(169,000)	482,000
Net (gain) loss	2,315,000	3,404,000	(4,986,000)
Amortization of loss	(898,000)	(659,000)	(1,206,000)
Total amount recognized in other comprehensive income	1,417,000	2,745,000	(6,192,000)
Total recognized in net periodic benefit cost and other comprehensive income	1,265,000	2,576,000	(5,710,000)
Retiree Medical Plan
Change in benefit obligation:
Benefit obligation at beginning of year	436,000	444,000	518,000
Interest cost	12,000	15,000	19,000
Actuarial (gain) loss	19,000	24,000	(41,000)
Benefits paid	(45,000)	(47,000)	(52,000)
Benefit obligation at end of year	422,000	436,000	444,000
Change in plan assets:
Employer contribution	45,000	47,000	52,000
Benefits paid	(45,000)	(47,000)	(52,000)
Funded status	$ (422,000)	$ (436,000)	$ (444,000)
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)	3.30%	3.00%	3.60%
Weighted-average assumptions used to determine net periodic pension cost
Discount rate (as a percent)	3.00%	3.60%	3.80%
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	$ 12,000	$ 15,000	$ 19,000
Recognized net actuarial loss	9,000	6,000	10,000
Net periodic pension expense	21,000	21,000	29,000
Net (gain) loss	19,000	24,000	(41,000)
Amortization of loss	(9,000)	(6,000)	(10,000)
Total amount recognized in other comprehensive income	10,000	18,000	(51,000)
Total recognized in net periodic benefit cost and other comprehensive income	31,000	39,000	(22,000)
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	10,000,000
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	27,000
One-Percentage Point Decrease, Effect on postretirement benefit obligation	(24,000)
Estimated future benefit payments (including expected future service as appropriate)
2016	42,000
2017	42,000
2018	41,000
2019	40,000
2020	39,000
2021-2025	166,000
Estimated future benefit payments	370,000
Expected contributions in the next fiscal year
Amount of expected contributions in the next fiscal year	42,000
Retiree Welfare Plan
Change in benefit obligation:
Benefit obligation at beginning of year	2,331,000	2,331,000	2,197,000
Interest cost	77,000	85,000
Participants' contributions		72,000
Actuarial (gain) loss	(105,000)	299,000
Benefits paid	(233,000)	(322,000)
less: federal subsidy on benefits paid	22,000
Benefit obligation at end of year	2,092,000	2,331,000	$ 2,331,000
Change in plan assets:
Employer contribution	233,000	250,000
Participants' contributions		72,000
Benefits paid	(233,000)	(322,000)
Funded status	$ (2,092,000)	$ (2,331,000)
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)	3.70%	3.45%
Weighted-average assumptions used to determine net periodic pension cost
Discount rate (as a percent)	3.45%	4.05%
Assumed health care cost trend rates
Health care cost trend rate assumed for next fiscal year (as a percent)	6.25%	6.50%
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	$ 77,000	$ 85,000
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	95,000
Estimated future benefit payments (including expected future service as appropriate)
2016	189,000
2017	190,000
2018	190,000
2019	190,000
2020	186,000
2021-2025	818,000
Estimated future benefit payments	1,763,000
Expected contributions in the next fiscal year
Amount of expected contributions in the next fiscal year	189,000,000
Retiree Welfare Plan | Other Comprehensive Income (Loss) [Member]
Change in benefit obligation:
Interest cost	77,000	84,000
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	77,000	84,000
Recognized net actuarial loss	115,000	96,000
Net periodic pension expense	192,000	180,000
Net (gain) loss	(106,000)	299,000
Amortization of loss	(115,000)	(96,000)
Total amount recognized in other comprehensive income	(221,000)	203,000
Total recognized in net periodic benefit cost and other comprehensive income	$ (29,000)	$ 383,000