Debt - Schedule of Interest Expense (Details) - Senior Notes - 5.50% convertible senior notes due 2021 - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Interest rate	5.50%		5.50%
Contractual interest coupon	$ 2.9	$ 2.9
Amortization of debt discount	1.6	1.5
Amortization of debt issuance costs	0.3	0.3
Total	$ 4.8	$ 4.7