Non-pension Post-retirement Benefits (Change in Accumulated Benefit Obligation) (Details) - Non-pension Post-retirement Benefit Plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in projected benefit obligation:
Benefit obligation, beginning of year	$ 60,265	$ 60,185
Service cost	632	798	$ 856
Interest cost	2,148	2,659	2,589
Plan participants' contributions	525	528
Actuarial (gain) loss	(5,591)	44
Exchange rate fluctuations	90	46
Benefits paid	(4,126)	(3,995)
Benefit obligation, end of year	53,943	60,265	60,185
Funded status and accrued benefit cost	(53,943)	(60,265)
U.S. Plans
Change in projected benefit obligation:
Benefit obligation, beginning of year	58,921	58,795
Service cost	631	797	855
Interest cost	2,104	2,608	2,537
Plan participants' contributions	525	528
Actuarial (gain) loss	(5,483)	146
Exchange rate fluctuations	0	0
Benefits paid	(4,050)	(3,953)
Benefit obligation, end of year	52,648	58,921	58,795
Funded status and accrued benefit cost	(52,648)	(58,921)
Non-U.S. Plans
Change in projected benefit obligation:
Benefit obligation, beginning of year	1,344	1,390
Service cost	1	1	1
Interest cost	44	51	52
Plan participants' contributions	0	0
Actuarial (gain) loss	(108)	(102)
Exchange rate fluctuations	90	46
Benefits paid	(76)	(42)
Benefit obligation, end of year	1,295	1,344	$ 1,390
Funded status and accrued benefit cost	$ (1,295)	$ (1,344)