Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined Benefits Plan Estimated Future Employer Contributions In Next Fiscal Year			$ 2,300
Post-Retirement Health Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefits Plan Estimated Future Employer Contributions In Next Fiscal Year			$ 264
Service cost	$ 10	$ 13
Interest cost	33	43
Expected return on plan assets	0	0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0	0
Net amortization of net (gain) loss	0	0
Net Periodic Benefit Cost	43	56
Defined Benefit Plan, Plan Assets, Contributions by Employer	61
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	347	358
Interest cost	798	905
Expected return on plan assets	(991)	(985)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0	0
Net amortization of net (gain) loss	362	301
Net Periodic Benefit Cost	516	$ 579
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 223