Pension and Non-pension Post-retirement Benefits (Net Benefit Costs) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,042	$ 1,271
Interest cost	4,151	3,928
Expected return on plan assets	(5,193)	(5,660)
Amortization of unrecognized:
Prior service cost (credit)	(50)	(51)
Actuarial loss / (gain)	1,190	1,796
Pension expense or non-pension post-retirement benefit expense	1,140	1,284
Defined Benefit Plan, Contributions [Abstract]
Employer contributions made to defined benefit plans	1,300
Estimated employer contributions to defined benefit plans in remainder of 2019	2,100
Defined Benefit Pension Plan | Domestic Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	783	979
Interest cost	3,382	3,165
Expected return on plan assets	(5,193)	(5,660)
Amortization of unrecognized:
Prior service cost (credit)	0	0
Actuarial loss / (gain)	1,087	1,637
Pension expense or non-pension post-retirement benefit expense	59	121
Defined Benefit Pension Plan | Foreign Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	259	292
Interest cost	769	763
Expected return on plan assets	0	0
Amortization of unrecognized:
Prior service cost (credit)	(50)	(51)
Actuarial loss / (gain)	103	159
Pension expense or non-pension post-retirement benefit expense	1,081	1,163
Non-Pension Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	110	151
Interest cost	478	466
Amortization of unrecognized:
Prior service cost (credit)	(70)	(71)
Actuarial loss / (gain)	(100)	(68)
Pension expense or non-pension post-retirement benefit expense	418	478
Defined Benefit Plan, Contributions [Abstract]
Employer contributions made to defined benefit plans	1,800
Estimated employer contributions to defined benefit plans in year 2019	4,000
Non-Pension Post-retirement Benefit Plans | Domestic Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	110	151
Interest cost	469	456
Amortization of unrecognized:
Prior service cost (credit)	(70)	(71)
Actuarial loss / (gain)	(82)	(52)
Pension expense or non-pension post-retirement benefit expense	427	484
Non-Pension Post-retirement Benefit Plans | Foreign Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	9	10
Amortization of unrecognized:
Prior service cost (credit)	0	0
Actuarial loss / (gain)	(18)	(16)
Pension expense or non-pension post-retirement benefit expense	$ (9)	$ (6)