Retirement Benefits - Components of Net Periodic Benefit Cost for Defined Benefit Plans and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 167	$ 152	$ 335	$ 304
Interest cost	203	204	406	408
Net amortization	(184)	(199)	(368)	(397)
Net periodic benefit cost	186	157	373	315
U.S. | Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	255	254	509	508
Interest cost	653	661	1,306	1,321
Expected return on plan assets	(983)	(944)	(1,966)	(1,888)
Net amortization	685	641	1,370	1,283
Net periodic benefit cost	610	612	1,219	1,224
Non-U.S. | Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	527	494	1,072	976
Interest cost	351	317	712	625
Expected return on plan assets	(282)	(270)	(572)	(534)
Net amortization	322	395	652	777
Net periodic benefit cost	$ 918	$ 936	$ 1,864	$ 1,844