Debt and Credit Facility - Amortization and Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Debt issuance amortization			$ 12.8	$ 12.1
Debt discount amortization			90.9	85.2
Other interest expense	$ 1.3	$ 0.6	2.6	1.5
Total interest expense	119.7	137.6	$ 381.9	366.7
2017 Senior Convertible Debt
Debt Instrument [Line Items]
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)			7 years 1 month 6 days
2015 Senior Convertible Debt
Debt Instrument [Line Items]
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)			5 years 1 month 6 days
2017 Junior Convertible Debt
Debt Instrument [Line Items]
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)			17 years 1 month 6 days
Total Senior Secured Indebtedness
Debt Instrument [Line Items]
Debt issuance amortization	3.3	3.6	$ 9.9	9.4
Debt discount amortization	0.7	0.7	2.1	1.5
Interest expense	64.1	84.2	217.7	210.1
Total interest expense on Senior Secured Indebtedness	68.1	88.5	229.7	221.0
Convertible Debt
Debt Instrument [Line Items]
Debt issuance amortization	1.0	0.9	2.9	2.7
Debt discount amortization	30.0	28.3	88.8	83.7
Interest expense	19.3	19.3	57.9	57.8
Total interest expense on Senior Secured Indebtedness	$ 50.3	$ 48.5	$ 149.6	$ 144.2