Debt - Schedule of Interest Expense (Details) - Senior Notes - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
The Notes
Debt Instrument [Line Items]
Contractual interest coupon	$ 0.0	$ 11.9	$ 13.8	$ 35.5
Amortization of debt issuance costs	0.0	0.6	0.7	1.8
Total	0.0	12.5	14.5	37.3
2021 Notes
Debt Instrument [Line Items]
Contractual interest coupon	1.2	1.8	3.5	7.7
Amortization of debt discount	0.7	1.2	2.3	4.7
Amortization of debt issuance costs	0.2	0.2	0.4	0.8
Total	$ 2.1	$ 3.2	$ 6.2	$ 13.2