Pension Plans and Postretirement Benefit Plan (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Net periodic benefit cost for pension plans and the postretirement benefit plan
Service Cost	$ 0	$ 0	$ 0	$ 0
Interest Cost	1,161	1,210	3,483	3,628
Expected Return on Plan Assets	(1,238)	(1,282)	(3,714)	(3,844)
Amortization of Prior Service Credit	0	0	0	0
Amortization of Net Actuarial Losses (Gains)	433	389	1,298	1,166
Net Periodic Benefit Cost	356	317	1,067	950
Employer Contributions	10,200		10,400
Estimated Future Employer Contributions in Current Fiscal Year			10,500
Postretirement Benefits
Net periodic benefit cost for pension plans and the postretirement benefit plan
Service Cost	123	137	369	411
Interest Cost	272	294	816	882
Expected Return on Plan Assets	0	0	0	0
Amortization of Prior Service Credit	(81)	(81)	(242)	(242)
Amortization of Net Actuarial Losses (Gains)	(111)	(75)	(331)	(226)
Net Periodic Benefit Cost	203	$ 275	612	$ 825
Employer Contributions	$ 200		700
Estimated Future Employer Contributions in Current Fiscal Year			$ 1,000