Debt and Credit Facility - Amortization and Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Debt Instrument [Line Items]
Debt issuance amortization	$ 16.5	$ 6.6	$ 4.5
Debt discount amortization	114.6	106.1	56.1
Other interest expense	2.9	2.4	0.8
Total interest expense	502.9	199.0	146.3
Senior Secured Indebtedness
Debt Instrument [Line Items]
Debt issuance amortization	12.9	3.1	2.4
Debt discount amortization	2.2	0.0	0.0
Interest expense	291.8	6.6	40.4
Total interest expense on Senior Secured Indebtedness	306.9	9.7	42.8
Convertible Debt
Debt Instrument [Line Items]
Debt issuance amortization	3.6	3.5	2.1
Debt discount amortization	112.4	106.1	56.1
Interest expense	77.1	77.3	44.5
Total interest expense on Senior Secured Indebtedness	$ 193.1	$ 186.9	$ 102.7
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 10 months 17 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 10 months 17 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 10 months 17 days