Debt - Unamortized Discount and Debt Issuance Costs (Details) - USD ($) $ in Millions	Sep. 30, 2022	Mar. 31, 2022
Debt Instrument [Line Items]
Total unamortized discount	$ (14.1)	$ (124.6)
Total debt issuance costs	(21.1)	(25.2)
Senior Indebtedness | 4.333% 2023 Notes
Debt Instrument [Line Items]
Total unamortized discount	(0.8)	(1.3)
Total debt issuance costs	(1.6)	(2.9)
Senior Indebtedness | 2.670% 2023 Notes
Debt Instrument [Line Items]
Total unamortized discount	(0.9)	(1.3)
Total debt issuance costs	(0.5)	(0.8)
Senior Indebtedness | 0.972% 2024 Notes
Debt Instrument [Line Items]
Total unamortized discount	(1.8)	(2.5)
Total debt issuance costs	(0.9)	(1.3)
Senior Indebtedness | 0.983% 2024 Notes
Debt Instrument [Line Items]
Total unamortized discount	(1.8)	(2.2)
Total debt issuance costs	(1.1)	(1.4)
Senior Indebtedness | 4.250% 2025 Notes
Debt Instrument [Line Items]
Total unamortized discount	(8.8)	(10.2)
Total debt issuance costs	(1.1)	(1.3)
Senior Convertible Debt | 2015 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	0.0	(3.7)
Total debt issuance costs	(0.1)	(0.1)
Senior Convertible Debt | 2017 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	0.0	(23.4)
Total debt issuance costs	(0.5)	(0.6)
Senior Convertible Debt | 2020 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	0.0	(75.3)
Total debt issuance costs	(5.8)	(6.2)
Junior Convertible Debt | 2017 Junior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	0.0	(4.7)
Total debt issuance costs	$ (0.1)	$ 0.0