Pension and Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	$ 988	$ 918
Defined Benefit Plan, Interest Cost	1,049	1,008
Defined Benefit Plan, Expected Return on Plan Assets	(1,290)	(1,326)
Defined Benefit Plan, Amortization of Losses	800	869
Defined Benefit Plan, Amortization of Prior Service Cost	(31)	(23)
Defined Benefit Plan, Net Periodic Benefit Cost	1,516	1,446
Defined Benefit Plan, Contributions by Employer	3,900
Pension Plans, Defined Benefit [Member] | 2018 Year [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Contributions In Current Fiscal Year	9,900
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	2	3
Defined Benefit Plan, Interest Cost	33	33
Defined Benefit Plan, Expected Return on Plan Assets	0	0
Defined Benefit Plan, Amortization of Losses	15	5
Defined Benefit Plan, Amortization of Prior Service Cost	0	0
Defined Benefit Plan, Net Periodic Benefit Cost	50	$ 41
Defined Benefit Plan, Contributions by Employer	100
Other Postretirement Benefit Plans, Defined Benefit [Member] | 2018 Year [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Contributions In Current Fiscal Year	$ 400