Pension and Non-pension Post-retirement Benefits (Net Benefit Costs) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,271	$ 1,326
Interest cost	3,928	4,087
Expected return on plan assets	(5,660)	(5,617)
Amortization of unrecognized:
Prior service cost (credit)	(51)	12
Actuarial loss / (gain)	1,796	1,490
Pension expense or non-pension post-retirement benefit expense	1,284	1,298
Defined Benefit Plan, Contributions [Abstract]
Employer contributions made to defined benefit plans	600
Estimated employer contributions to defined benefit plans in remainder of 2018	1,700
Defined Benefit Pension Plan | Domestic Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	979	1,075
Interest cost	3,165	3,450
Expected return on plan assets	(5,660)	(5,617)
Amortization of unrecognized:
Prior service cost (credit)	0	59
Actuarial loss / (gain)	1,637	1,352
Pension expense or non-pension post-retirement benefit expense	121	319
Defined Benefit Pension Plan | Foreign Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	292	251
Interest cost	763	637
Expected return on plan assets	0	0
Amortization of unrecognized:
Prior service cost (credit)	(51)	(47)
Actuarial loss / (gain)	159	138
Pension expense or non-pension post-retirement benefit expense	1,163	979
Non-Pension Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	151	220
Interest cost	466	592
Amortization of unrecognized:
Prior service cost (credit)	(71)	(50)
Actuarial loss / (gain)	(68)	12
Pension expense or non-pension post-retirement benefit expense	478	774
Defined Benefit Plan, Contributions [Abstract]
Employer contributions made to defined benefit plans	800
Estimated employer contributions to defined benefit plans in year 2018	4,300
Non-Pension Postretirement Benefit Plans | Domestic Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	151	220
Interest cost	456	581
Amortization of unrecognized:
Prior service cost (credit)	(71)	(50)
Actuarial loss / (gain)	(52)	25
Pension expense or non-pension post-retirement benefit expense	484	776
Non-Pension Postretirement Benefit Plans | Foreign Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	10	11
Amortization of unrecognized:
Prior service cost (credit)	0	0
Actuarial loss / (gain)	(16)	(13)
Pension expense or non-pension post-retirement benefit expense	$ (6)	$ (2)