Non-pension Post-retirement Benefits (Net Benefit Costs) (Details) - Non-pension Post-retirement Benefit Plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost (benefits earned during the period)	$ 632	$ 798	$ 856
Interest cost on projected benefit obligation	2,148	2,659	2,589
Amortization of unrecognized:
Prior service cost (credit)	(201)	140	140
Actuarial loss (gain)	(316)	34	536
Non-pension post-retirement benefit expense	2,263	3,631	4,121
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost (benefits earned during the period)	631	797	855
Interest cost on projected benefit obligation	2,104	2,608	2,537
Amortization of unrecognized:
Prior service cost (credit)	(201)	140	140
Actuarial loss (gain)	(257)	81	592
Non-pension post-retirement benefit expense	2,277	3,626	4,124
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost (benefits earned during the period)	1	1	1
Interest cost on projected benefit obligation	44	51	52
Amortization of unrecognized:
Prior service cost (credit)	0	0	0
Actuarial loss (gain)	(59)	(47)	(56)
Non-pension post-retirement benefit expense	$ (14)	$ 5	$ (3)