Amortization and Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Debt Instrument [Line Items]
Amortization of debt issuance costs	$ 6.6	$ 4.5	$ 4.0
Amortization of debt discount on convertible debt	106.1	56.1	48.0
Convertible Debt
Debt Instrument [Line Items]
Amortization of debt issuance costs	3.5	2.1	1.8
Amortization of debt discount on convertible debt	106.1	56.1	48.0
Coupon interest expense	77.3	44.5	40.2
Total	$ 186.9	$ 102.7	$ 90.0
2017 Senior Subordinated 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)	8 years 10 months 16 days
2015 Senior Subordinated 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)	6 years 10 months 16 days
2017 Junior Subordinated 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)	18 years 10 months 16 days