Pension and Non-pension Post-retirement Benefits (Net Benefit Costs) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,309	$ 1,160	$ 2,580	$ 2,486
Interest cost	3,883	4,143	7,811	8,230
Expected return on plan assets	(5,669)	(5,623)	(11,329)	(11,240)
Amortization of unrecognized:
Prior service cost (credit)	(49)	7	(100)	19
Actuarial loss / (gain)	1,753	1,417	3,549	2,907
Pension expense or non-pension post-retirement benefit expense	1,227	1,104	2,511	2,402
Defined Benefit Plan, Contributions [Abstract]
Employer contributions made to defined benefit plans	600		1,200
Estimated employer contributions to defined benefit plans in remainder of 2018	1,100		1,100
Defined Benefit Pension Plan | Domestic Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,025	883	2,004	1,958
Interest cost	3,142	3,442	6,307	6,892
Expected return on plan assets	(5,669)	(5,623)	(11,329)	(11,240)
Amortization of unrecognized:
Prior service cost (credit)	1	59	1	118
Actuarial loss / (gain)	1,599	1,265	3,236	2,617
Pension expense or non-pension post-retirement benefit expense	98	26	219	345
Defined Benefit Pension Plan | Foreign Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	284	277	576	528
Interest cost	741	701	1,504	1,338
Expected return on plan assets	0	0	0	0
Amortization of unrecognized:
Prior service cost (credit)	(50)	(52)	(101)	(99)
Actuarial loss / (gain)	154	152	313	290
Pension expense or non-pension post-retirement benefit expense	1,129	1,078	2,292	2,057
Non-Pension Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	151	96	302	316
Interest cost	465	482	931	1,074
Amortization of unrecognized:
Prior service cost (credit)	(70)	(51)	(141)	(101)
Actuarial loss / (gain)	(70)	(167)	(138)	(155)
Pension expense or non-pension post-retirement benefit expense	476	360	954	1,134
Defined Benefit Plan, Contributions [Abstract]
Employer contributions made to defined benefit plans	1,100		1,900
Estimated employer contributions to defined benefit plans in year 2018	4,300		4,300
Non-Pension Post-retirement Benefit Plans | Domestic Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	151	96	302	316
Interest cost	455	471	911	1,052
Amortization of unrecognized:
Prior service cost (credit)	(70)	(51)	(141)	(101)
Actuarial loss / (gain)	(53)	(154)	(105)	(129)
Pension expense or non-pension post-retirement benefit expense	483	362	967	1,138
Non-Pension Post-retirement Benefit Plans | Foreign Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	0
Interest cost	10	11	20	22
Amortization of unrecognized:
Prior service cost (credit)	0	0	0	0
Actuarial loss / (gain)	(17)	(13)	(33)	(26)
Pension expense or non-pension post-retirement benefit expense	$ (7)	$ (2)	$ (13)	$ (4)