Retirement Plans and Postretirement Medical Benefits (Components of Net Periodic Benefit Cost for Defined Benefit Pension Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Nonpension postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,727	$ 2,046	$ 2,455
Interest cost	7,100	7,969	8,799
Amortization of prior service credit	297	297	297
Amortization of net actuarial loss	3,600	3,615	7,528
Net periodic benefit (income) cost	12,724	13,927	19,079
United States | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	132	105	134
Interest cost	68,611	73,699	74,331
Expected return on plan assets	(97,656)	(101,918)	(104,004)
Amortization of net transition asset	0	0	0
Amortization of prior service credit	(60)	(60)	(60)
Amortization of net actuarial loss	28,954	27,220	29,272
Special termination benefits	0	0	0
Settlement	0	2,109	1,243
Net periodic benefit (income) cost	(19)	1,155	916
Foreign | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2,274	2,148	2,229
Interest cost	18,836	21,886	24,261
Expected return on plan assets	(32,242)	(32,615)	(35,421)
Amortization of net transition asset	(8)	(8)	(9)
Amortization of prior service credit	(71)	(73)	(66)
Amortization of net actuarial loss	8,052	5,264	5,926
Special termination benefits	0	52	79
Settlement	0	110	0
Net periodic benefit (income) cost	$ (3,159)	$ (3,236)	$ (3,001)