Retirement Benefits - Pretax Amounts Recognized in Accumulated Other Comprehensive Income Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Benefit Plan | U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	$ 86	$ 110
Net loss	27,789	27,860
Total	27,875	27,970
Net gain (loss) in current year	(2,471)
Amortization of prior service cost (credit)	24
Amortization of net loss (gain)	2,542
Exchange rate effect on amounts in OCI	0
Total	95
Pension Benefit Plan | Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	18	77
Net loss	17,986	17,643
Total	18,004	17,720
Net gain (loss) in current year	318
Amortization of prior service cost (credit)	3
Amortization of net loss (gain)	2,040
Exchange rate effect on amounts in OCI	(2,645)
Total	(284)
Other Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	(483)	(849)
Net loss	(2,866)	(3,852)
Total	(3,349)	$ (4,701)
Net gain (loss) in current year	(592)
Amortization of prior service cost (credit)	(366)
Amortization of net loss (gain)	(429)
Exchange rate effect on amounts in OCI	35
Total	$ (1,352)