Debt - Unamortized Discount and Debt Issuance Costs (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 01, 2020	Mar. 31, 2020	May 31, 2018
Debt Instrument [Line Items]
Total unamortized discount	$ (399.6)		$ (1,043.2)
Total debt issuance costs	(38.0)		(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 | 3.922% 2021 Notes
Debt Instrument [Line Items]
Total unamortized discount	(0.8)		(2.1)
Total debt issuance costs	(1.7)		(4.8)
Senior Indebtedness | 4.333% 2023 Notes
Debt Instrument [Line Items]
Total unamortized discount	(2.7)		(3.5)
Total debt issuance costs	(5.9)		(7.7)
Senior Indebtedness | 2.670% 2023 Notes
Debt Instrument [Line Items]
Total unamortized discount	(2.5)		0.0
Total debt issuance costs	(1.4)		0.0
Senior Indebtedness | 0.972% 2024 Notes
Debt Instrument [Line Items]
Total unamortized discount	(4.1)	$ (4.2)	0.0
Total debt issuance costs	(2.2)		0.0
Senior Indebtedness | 4.250% 2025 Notes
Debt Instrument [Line Items]
Total unamortized discount	(13.5)		0.0
Total debt issuance costs	(1.8)		0.0
Senior Indebtedness | 2020 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(110.0)
Senior Convertible Debt | 2015 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(33.3)		(192.9)
Total debt issuance costs	(1.2)		(7.0)
Senior Convertible Debt | 2017 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(100.9)		(504.2)
Total debt issuance costs	(2.6)		(13.0)
Senior Convertible Debt | 2020 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(108.0)		0.0
Total debt issuance costs	(8.9)		0.0
Junior Convertible Debt | 2017 Junior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(133.8)		(337.4)
Total debt issuance costs	$ (1.2)		$ (3.1)