Debt - Schedule of Interest Expense (Details) - Senior Notes - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
5.50% convertible senior notes due 2021
Debt Instrument [Line Items]
Contractual interest coupon	$ 2.9	$ 2.9
Amortization of debt discount	1.7	1.6
Amortization of debt issuance costs	0.3	0.3
Total	4.9	4.8
2022 Notes
Debt Instrument [Line Items]
Contractual interest coupon	11.8	11.8
Amortization of debt issuance costs	0.6	0.6
Total	$ 12.4	$ 12.4