Pension and Non-pension Postretirement Benefits (Net Benefit Costs) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,265	$ 1,242	$ 3,751	$ 3,736
Interest cost	4,170	4,403	12,400	13,227
Expected return on plan assets	(5,619)	(5,757)	(16,859)	(17,272)
Amortization of unrecognized:
Prior service cost (credit)	5	12	24	37
Actuarial loss / (gain)	1,464	1,480	4,371	4,021
Settlement charge			0	212
Pension expense or non-pension postretirement benefit expense	1,285	1,380	3,687	3,961
Defined Benefit Plan, Contributions [Abstract]
Employer contributions made to defined benefit plans	600		2,600
Estimated employer contributions to defined benefit plans in remainder of 2017	500		500
Defined Benefit Pension Plan | Domestic Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	979	929	2,937	2,788
Interest cost	3,445	3,740	10,337	11,222
Expected return on plan assets	(5,619)	(5,757)	(16,859)	(17,272)
Amortization of unrecognized:
Prior service cost (credit)	59	65	177	197
Actuarial loss / (gain)	1,308	1,068	3,925	3,204
Settlement charge			0	42
Pension expense or non-pension postretirement benefit expense	172	45	517	181
Defined Benefit Pension Plan | Foreign Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	286	313	814	948
Interest cost	725	663	2,063	2,005
Expected return on plan assets	0	0	0	0
Amortization of unrecognized:
Prior service cost (credit)	(54)	(53)	(153)	(160)
Actuarial loss / (gain)	156	412	446	817
Settlement charge			0	170
Pension expense or non-pension postretirement benefit expense	1,113	1,335	3,170	3,780
Non-Pension Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	157	200	474	599
Interest cost	536	663	1,610	1,991
Amortization of unrecognized:
Prior service cost (credit)	(50)	35	(151)	105
Actuarial loss / (gain)	(78)	11	(233)	29
Pension expense or non-pension postretirement benefit expense	565	909	1,700	2,724
Defined Benefit Plan, Contributions [Abstract]
Employer contributions made to defined benefit plans	1,000		2,600
Estimated employer contributions to defined benefit plans in year 2017	4,000		4,000
Non-Pension Postretirement Benefit Plans | Domestic Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	157	200	473	598
Interest cost	526	652	1,578	1,956
Amortization of unrecognized:
Prior service cost (credit)	(50)	35	(151)	105
Actuarial loss / (gain)	(65)	21	(194)	61
Pension expense or non-pension postretirement benefit expense	568	908	1,706	2,720
Non-Pension Postretirement Benefit Plans | Foreign Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	1	1
Interest cost	10	11	32	35
Amortization of unrecognized:
Prior service cost (credit)	0	0	0	0
Actuarial loss / (gain)	(13)	(10)	(39)	(32)
Pension expense or non-pension postretirement benefit expense	$ (3)	$ 1	$ (6)	$ 4