Debt - Unamortized Discount and Debt Issuance Costs (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Debt Instrument [Line Items]
Total unamortized discount	$ (273.0)		$ (1,043.2)
Total debt issuance costs	(32.3)		(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 | 3.922% 2021 Notes
Debt Instrument [Line Items]
Total unamortized discount	(0.3)		(2.1)
Total debt issuance costs	(0.7)		(4.8)
Senior Indebtedness | 4.333% 2023 Notes
Debt Instrument [Line Items]
Total unamortized discount	(2.4)		(3.5)
Total debt issuance costs	(5.3)		(7.7)
Senior Indebtedness | 2.670% 2023 Notes
Debt Instrument [Line Items]
Total unamortized discount	(2.3)		0.0
Total debt issuance costs	(1.3)		0.0
Senior Indebtedness | 0.972% 2024 Notes
Debt Instrument [Line Items]
Total unamortized discount	(3.8)		0.0
Total debt issuance costs	(2.0)		0.0
Senior Indebtedness | 4.250% 2025 Notes
Debt Instrument [Line Items]
Total unamortized discount	(12.8)		0.0
Total debt issuance costs	(1.7)		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	(20.1)		(192.9)
Total debt issuance costs	(0.7)		(7.0)
Senior Convertible Debt | 2017 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(71.3)		(504.2)
Total debt issuance costs	(1.8)		(13.0)
Senior Convertible Debt | 2020 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(101.6)		0.0
Total debt issuance costs	(8.3)		0.0
Junior Convertible Debt | 2017 Junior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(58.4)		(337.4)
Total debt issuance costs	$ (0.5)		$ (3.1)