Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pensions [Member]
Pension and other postretirement benefits expenses
Service cost	$ 1,459	$ 1,318	$ 3,102	$ 2,702
Interest cost	4,666	4,945	9,287	9,781
Expected return on plan assets	(6,816)	(7,667)	(13,676)	(15,210)
Amortization of prior service cost (credit)	111	55	220	105
Amortization of actuarial losses	2,663	2,856	5,143	5,412
Curtailment gain	(7,217)	0	(7,217)	0
Settlement gain	(230)	0	(491)	0
Net periodic benefit cost	(5,364)	1,507	(3,632)	2,790
Pensions [Member] | Domestic Plan [Member] | Qualified Plan [Member]
Pension and other postretirement benefits expenses
Discretionary contributions			5,000
Pensions [Member] | Domestic Plan [Member] | Nonqualified Plan [Member]
Pension and other postretirement benefits expenses
Expected company contributions in 2017	5,000		5,000
Other Postretirement Benefits [Member]
Pension and other postretirement benefits expenses
Service cost	20	27	42	61
Interest cost	392	431	781	884
Amortization of prior service cost (credit)	(17)	(94)	(34)	(187)
Amortization of actuarial losses	70	91	139	267
Net periodic benefit cost	$ 465	$ 455	$ 928	$ 1,025