Debt and Credit Facility - Amortization and Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Amortization of debt issuance costs			$ 8.4	$ 3.3
Amortization of debt discount on convertible debt			55.4	52.3
Total interest expense on convertible debt	$ 103.4	$ 46.6	170.7	92.9
Convertible Debt
Debt Instrument [Line Items]
Amortization of debt issuance costs	0.9	0.9	1.8	1.8
Amortization of debt discount on convertible debt	27.9	26.3	55.4	52.3
Coupon interest expense	19.3	19.4	38.6	38.8
Total interest expense on convertible debt	48.1	46.6	95.8	92.9
Other Long-Term Debt
Debt Instrument [Line Items]
Amortization of debt issuance costs	3.6	0.0	4.9	0.0
Amortization of debt discount on convertible debt	51.7	0.0	70.0	0.0
Coupon interest expense	$ 55.3	$ 0.0	$ 74.9	$ 0.0