Retirement Benefits - Components of Net Periodic Benefit Cost for Defined Benefit Plans and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 167	$ 152	$ 502	$ 456
Interest cost	202	203	608	611
Net amortization	(185)	(198)	(553)	(595)
Net periodic benefit cost	184	157	557	472
U.S. | Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	156	224	665	732
Interest cost	669	687	1,975	2,008
Expected return on plan assets	(991)	(986)	(2,957)	(2,874)
Net amortization	664	641	2,034	1,924
Net periodic benefit cost	498	566	1,717	1,790
Non-U.S. | Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	520	520	1,592	1,496
Interest cost	341	332	1,053	957
Expected return on plan assets	(276)	(277)	(848)	(811)
Net amortization	312	418	964	1,195
Net periodic benefit cost	$ 897	$ 993	$ 2,761	$ 2,837