Debt and Credit Facility Amortization and Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Debt Instrument [Line Items]
Amortization of debt issuance costs	$ 3.0	$ 1.6
Amortization of debt discount on convertible debt	27.5	25.9
Total interest expense on convertible debt	$ 67.3	46.2
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)	8 years 7 months 16 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)	6 years 7 months 16 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)	18 years 7 months 16 days
Convertible Debt
Debt Instrument [Line Items]
Amortization of debt issuance costs	$ 0.9	0.9
Amortization of debt discount on convertible debt	27.5	25.9
Coupon interest expense	19.3	19.4
Total interest expense on convertible debt	47.7	46.2
Other Long-Term Debt
Debt Instrument [Line Items]
Amortization of debt issuance costs	1.3	0.0
Amortization of debt discount on convertible debt	18.3	0.0
Coupon interest expense	$ 19.6	$ 0.0