Debt - Unamortized Discount and Debt Issuance Costs (Details) - USD ($) $ in Millions	Sep. 30, 2020	Mar. 31, 2020	May 31, 2018
Debt Instrument [Line Items]
Total unamortized discount	$ (641.6)	$ (1,043.2)
Total debt issuance costs	(49.2)	(67.9)
Bridge Loan Facility
Debt Instrument [Line Items]
Total unamortized discount	0.0	(3.1)
Total debt issuance costs	0.0	(3.1)
Senior Indebtedness
Debt Instrument [Line Items]
Total unamortized discount			$ (10.5)
Total debt issuance costs			$ (24.4)
Senior Indebtedness | 4.333% 2023 Notes
Debt Instrument [Line Items]
Total unamortized discount	(2.9)	(3.5)
Total debt issuance costs	(6.6)	(7.7)
Senior Indebtedness | 3.922% 2021 Notes
Debt Instrument [Line Items]
Total unamortized discount	(1.2)	(2.1)
Total debt issuance costs	(2.7)	(4.8)
Senior Indebtedness | 2.670% 2023 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	(14.2)	0.0
Total debt issuance costs	(1.9)	0.0
Senior Convertible Debt | 2017 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(239.0)	(504.2)
Total debt issuance costs	(6.1)	(13.0)
Senior Convertible Debt | 2015 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(49.4)	(192.9)
Total debt issuance costs	(1.8)	(7.0)
Junior Convertible Debt | 2017 Junior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(332.2)	(337.4)
Total debt issuance costs	$ (3.0)	$ (3.1)