Pension and Postretirement Benefits (Net Benefit Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Postretirement Medical Benefits
Components of the net periodic cost for retirement and postretirement benefits
Service cost	$ 0	$ 1	$ 2
Interest cost	919	1,154	1,155
Amortization of unrecognized net loss	(275)	(355)	(539)
Amortization of unrecognized prior service (benefit) cost	122	122	122
U.S. net pension expense	766	922	740
Domestic Plan
Components of the net periodic cost for retirement and postretirement benefits
Service cost	22,257	22,583	24,113
Interest cost	16,878	19,072	17,072
Expected return on plan assets	(24,505)	(23,997)	(24,185)
Settlement loss and other	(216)	91	0
Amortization of unrecognized net loss	6,021	4,999	9,178
Amortization of unrecognized prior service (benefit) cost	368	(634)
Amortization of unrecognized prior service cost	112	488	509
U.S. net pension expense	20,547	23,236	26,687
Foreign Plan
Components of the net periodic cost for retirement and postretirement benefits
Service cost	7,247	7,131	7,832
Interest cost	9,320	11,623	11,770
Expected return on plan assets	(8,834)	(10,013)	(11,693)
Settlement loss and other	2,434	780	570
Amortization of unrecognized net loss	3,741	4,751	4,949
Amortization of unrecognized prior service (benefit) cost	(4)	4	(12)
U.S. net pension expense	$ 13,904	$ 14,276	$ 13,416