Retirement Benefits - Components of Net Periodic Benefit Cost for Defined Benefit Plans and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefit Plan | U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 976	$ 1,016	$ 1,279
Interest cost		2,677	3,043	3,770
Expected return on plan assets		(3,832)	(4,777)	(4,910)
Settlement loss recognized	$ 3,500	0	3,339	0
Net amortization		2,566	3,226	3,422
Net periodic benefit cost		2,387	5,847	3,561
Pension Benefit Plan | Non-U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		1,975	1,627	1,506
Interest cost		1,283	1,429	1,734
Expected return on plan assets		(1,088)	(993)	(1,114)
Settlement loss recognized		234	215	0
Net amortization		1,809	1,008	1,931
Net periodic benefit cost		4,213	3,286	4,057
Other Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		610	601	673
Interest cost		818	811	833
Net amortization		(795)	(705)	(414)
Net periodic benefit cost		$ 633	$ 707	$ 1,092