Debt - Schedule of Interest Expense, Debt (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2018
Debt Instrument [Line Items]
Accretion of debt discount			$ 37.0	$ 11.3
Senior convertible notes due 2022
Debt Instrument [Line Items]
Contractual coupon interest	$ 0.8	$ 0.8	2.3	2.3
Accretion of debt discount	3.5	3.4	10.4	10.0
Amortization of debt issuance costs	0.4	0.4	1.2	1.2
Total interest expense recognized on senior notes	$ 4.7	4.6	$ 13.9	13.5
Effective interest rate	5.10%		5.10%		5.10%
Senior convertible notes due 2023
Debt Instrument [Line Items]
Contractual coupon interest	$ 1.6	0.0	$ 4.8	0.0
Accretion of debt discount	8.0	0.0	23.7	0.0
Amortization of debt issuance costs	0.5	0.0	1.5	0.0
Total interest expense recognized on senior notes	$ 10.1	0.0	$ 30.0	0.0
Effective interest rate	5.60%		5.60%		0.00%
Senior Notes \| Zero Point Seven Five Percent Senior Convertible Notes
Debt Instrument [Line Items]
Total interest expense recognized on senior notes	$ 14.8	$ 4.6	$ 43.9	$ 13.5

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of amortization expense attributable to debt issuance costs.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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