Retirement Benefits - Components of Net Periodic Benefit Cost for Defined Benefit Plans and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 168	$ 152
Interest cost	203	204
Net amortization	(184)	(198)
Net periodic benefit cost	187	158
U.S. | Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	254	254
Interest cost	653	660
Expected return on plan assets	(983)	(944)
Net amortization	685	642
Net periodic benefit cost	609	612
Non-U.S. | Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	545	482
Interest cost	361	308
Expected return on plan assets	(290)	(264)
Net amortization	330	382
Net periodic benefit cost	$ 946	$ 908