Employee Benefits Plans (Details 10) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amortization of:
Postretirement benefit income	$ 56,833	$ 61,215	$ 41,704
Other Postretirement Benefits
Other Postretirement benefit expense
Service cost	191	215	363
Interest cost	833	906	1,097
Amortization of:
Net actuarial gain	(1,796)	(1,989)	(1,773)
Prior service cost (credit)	(231)	(231)	(1,083)
Postretirement benefit income	(1,003)	(1,099)	(1,396)
United States | Other Postretirement Benefits
Amortization of:
Postretirement benefit income	(1,308)	(1,429)	(1,887)
Foreign Plan | Other Postretirement Benefits
Amortization of:
Postretirement benefit income	$ 305	$ 330	$ 491