Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (25,300,000)	$ 24,400,000
Total other comprehensive income, net of tax	1,700,000	75,500,000
Ending balance	(26,700,000)	(25,300,000)
Liability adjustments for defined benefit plans	0
Accumulated foreign currency translation losses	800,000
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(20,600,000)	(96,100,000)
Other comprehensive (loss) income before reclassifications, net of tax	300,000
Amounts reclassified from accumulated other comprehensive loss, net of tax	1,400,000
Total other comprehensive income, net of tax	1,700,000
Ending balance	(18,900,000)	(20,600,000)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(19,600,000)
Other comprehensive (loss) income before reclassifications, net of tax	0
Amounts reclassified from accumulated other comprehensive loss, net of tax	1,400,000
Total other comprehensive income, net of tax	1,400,000
Ending balance	(18,200,000)	(19,600,000)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(1,000,000)
Other comprehensive (loss) income before reclassifications, net of tax	300,000
Amounts reclassified from accumulated other comprehensive loss, net of tax	0
Total other comprehensive income, net of tax	300,000
Ending balance	$ (700,000)	$ (1,000,000)