Debt and Credit Facility - Amortization and Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Debt issuance amortization			$ 8.5	$ 7.5
Debt discount amortization			60.2	56.3
Other interest expense	$ 0.8	$ 0.3	1.3	0.7
Total interest expense	129.6	138.6	$ 262.2	229.0
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 4 months 24 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 4 months 24 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 4 months 24 days
Total Senior Secured Indebtedness
Debt Instrument [Line Items]
Debt issuance amortization	3.3	3.8	$ 6.6	5.7
Debt discount amortization	0.7	0.7	1.4	0.9
Interest expense	74.9	85.7	153.6	125.9
Total interest expense on Senior Secured Indebtedness	78.9	90.2	161.6	132.5
Convertible Debt
Debt Instrument [Line Items]
Debt issuance amortization	1.0	0.9	1.9	1.8
Debt discount amortization	29.6	27.9	58.8	55.4
Interest expense	19.3	19.3	38.6	38.6
Total interest expense on Senior Secured Indebtedness	$ 49.9	$ 48.1	$ 99.3	$ 95.8