Pensions and Other Postretirement Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 699	$ 651
Interest cost	1,820	1,828
Expected return on assets	(2,247)	(1,993)
Amortization of prior service cost/(credit)	8	9
Amortization of net actuarial loss	558	645
Net periodic benefit cost	838	1,140
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	58	61
Interest cost	507	553
Expected return on assets
Amortization of prior service cost/(credit)	(1,122)	(1,122)
Amortization of net actuarial loss	739	702
Net periodic benefit cost	$ 182	$ 194