Pension Plans and Postretirement Benefit Plan (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits
Net periodic benefit cost for pension plans and the postretirement benefit plan
Service Cost	$ 0	$ 0
Interest Cost	1,041	1,161
Expected Return on Plan Assets	(1,282)	(1,238)
Amortization of Prior Service Credit	0	0
Amortization of Net Actuarial Losses (Gains)	498	433
Net Periodic Benefit Cost	257	356
Employer Contributions	100
Estimated Future Employer Contributions in Current Fiscal Year	500
Postretirement Benefits
Net periodic benefit cost for pension plans and the postretirement benefit plan
Service Cost	115	123
Interest Cost	235	272
Expected Return on Plan Assets	0	0
Amortization of Prior Service Credit	(142)	(81)
Amortization of Net Actuarial Losses (Gains)	(62)	(110)
Net Periodic Benefit Cost	146	$ 204
Employer Contributions	300
Estimated Future Employer Contributions in Current Fiscal Year	$ 900