Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined Benefits Plan Estimated Future Employer Contributions In Next Fiscal Year					$ 2,500
Post-Retirement Health Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefits Plan Estimated Future Employer Contributions In Next Fiscal Year					$ 260
Service cost	$ 13	$ 14	$ 40	$ 41
Interest cost	43	46	129	139
Expected return on plan assets	0	0	0	0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0	0	0	0
Net amortization of net (gain) loss	0	0	0	0
Net Periodic Benefit Cost	56	60	169	180
Defined Benefit Plan, Plan Assets, Contributions by Employer			167
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	358	470	1,074	1,411
Interest cost	905	932	2,716	2,797
Expected return on plan assets	(985)	(857)	(2,955)	(2,572)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0	0	1	1
Net amortization of net (gain) loss	301	484	903	1,451
Net Periodic Benefit Cost	$ 579	$ 1,029	1,739	$ 3,088
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 1,900