Debt - Unamortized Discount and Debt Issuance Costs (Details) - USD ($) $ in Millions	Sep. 30, 2021	Mar. 31, 2021
Debt Instrument [Line Items]
Total unamortized discount	$ (178.9)	$ (273.0)
Total debt issuance costs	(26.3)	(32.3)
Senior Indebtedness | 3.922% 2021 Notes
Debt Instrument [Line Items]
Total unamortized discount	0.0	(0.3)
Total debt issuance costs	0.0	(0.7)
Senior Indebtedness | 4.333% 2023 Notes
Debt Instrument [Line Items]
Total unamortized discount	(1.9)	(2.4)
Total debt issuance costs	(4.1)	(5.3)
Senior Indebtedness | 2.670% 2023 Notes
Debt Instrument [Line Items]
Total unamortized discount	(1.8)	(2.3)
Total debt issuance costs	(1.0)	(1.3)
Senior Indebtedness | 0.972% 2024 Notes
Debt Instrument [Line Items]
Total unamortized discount	(3.2)	(3.8)
Total debt issuance costs	(1.6)	(2.0)
Senior Indebtedness | 0.983% 2024 Notes
Debt Instrument [Line Items]
Total unamortized discount	(2.7)	0.0
Total debt issuance costs	(1.6)	0.0
Senior Indebtedness | 4.250% 2025 Notes
Debt Instrument [Line Items]
Total unamortized discount	(11.5)	(12.8)
Total debt issuance costs	(1.5)	(1.7)
Senior Convertible Debt | 2015 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(8.9)	(20.1)
Total debt issuance costs	(0.3)	(0.7)
Senior Convertible Debt | 2017 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(46.2)	(71.3)
Total debt issuance costs	(1.2)	(1.8)
Senior Convertible Debt | 2020 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(88.6)	(101.6)
Total debt issuance costs	(7.3)	(8.3)
Junior Convertible Debt | 2017 Junior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(14.1)	(58.4)
Total debt issuance costs	$ (0.1)	$ (0.5)