Pension and Non-pension Post-retirement Benefits (Net Benefit Costs) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,043	$ 1,309	$ 2,085	$ 2,580
Interest cost	4,154	3,883	8,305	7,811
Expected return on plan assets	(5,193)	(5,669)	(10,386)	(11,329)
Amortization of unrecognized:
Prior service cost (credit)	(51)	(49)	(101)	(100)
Actuarial loss / (gain)	1,193	1,753	2,383	3,549
Pension expense or non-pension post-retirement benefit expense	1,146	1,227	2,286	2,511
Defined Benefit Plan, Contributions [Abstract]
Employer contributions made to defined benefit plans	400		1,700
Estimated employer contributions to defined benefit plans in remainder of 2019	1,700		1,700
Defined Benefit Pension Plan | Domestic Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	783	1,025	1,566	2,004
Interest cost	3,382	3,142	6,764	6,307
Expected return on plan assets	(5,193)	(5,669)	(10,386)	(11,329)
Amortization of unrecognized:
Prior service cost (credit)	0	1	0	1
Actuarial loss / (gain)	1,088	1,599	2,175	3,236
Pension expense or non-pension post-retirement benefit expense	60	98	119	219
Defined Benefit Pension Plan | Foreign Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	260	284	519	576
Interest cost	772	741	1,541	1,504
Expected return on plan assets	0	0	0	0
Amortization of unrecognized:
Prior service cost (credit)	(51)	(50)	(101)	(101)
Actuarial loss / (gain)	105	154	208	313
Pension expense or non-pension post-retirement benefit expense	1,086	1,129	2,167	2,292
Non-Pension Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	112	151	222	302
Interest cost	458	465	936	931
Amortization of unrecognized:
Prior service cost (credit)	(71)	(70)	(141)	(141)
Actuarial loss / (gain)	(125)	(70)	(225)	(138)
Pension expense or non-pension post-retirement benefit expense	374	476	792	954
Defined Benefit Plan, Contributions [Abstract]
Employer contributions made to defined benefit plans	1,700		3,500
Estimated employer contributions to defined benefit plans in year 2019	5,500		5,500
Non-Pension Post-retirement Benefit Plans | Domestic Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	112	151	222	302
Interest cost	449	455	918	911
Amortization of unrecognized:
Prior service cost (credit)	(71)	(70)	(141)	(141)
Actuarial loss / (gain)	(106)	(53)	(188)	(105)
Pension expense or non-pension post-retirement benefit expense	384	483	811	967
Non-Pension Post-retirement Benefit Plans | Foreign Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	9	10	18	20
Amortization of unrecognized:
Prior service cost (credit)	0	0	0	0
Actuarial loss / (gain)	(19)	(17)	(37)	(33)
Pension expense or non-pension post-retirement benefit expense	$ (10)	$ (7)	$ (19)	$ (13)