Debt - Unamortized Discount and Debt Issuance Costs (Details) - USD ($) $ in Millions	Mar. 31, 2020	Mar. 27, 2020	Mar. 31, 2019	May 31, 2018	Feb. 28, 2017	Feb. 28, 2015
Debt Instrument [Line Items]
Total unamortized discount	$ (1,043.2)		$ (1,268.7)
Total debt issuance costs	(67.9)		(83.6)		$ (33.7)
2015 Senior Convertible Debt
Debt Instrument [Line Items]
Total debt issuance costs						$ (30.3)
Bridge Loan Facility
Debt Instrument [Line Items]
Total unamortized discount	(3.1)		0.0
Total debt issuance costs	(3.1)	$ (6.2)	0.0
Senior Notes
Debt Instrument [Line Items]
Total unamortized discount				$ (10.5)
Total debt issuance costs				$ (24.4)
Senior Notes | 2023 Notes
Debt Instrument [Line Items]
Total unamortized discount	(3.5)		(4.4)
Total debt issuance costs	(7.7)		(10.2)
Senior Notes | 2021 Notes
Debt Instrument [Line Items]
Total unamortized discount	(2.1)		(3.8)
Total debt issuance costs	(4.8)		(8.8)
Senior Convertible Debt | 2017 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(504.2)		(561.9)
Total debt issuance costs	(13.0)		(14.5)
Senior Convertible Debt | 2015 Senior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(192.9)		(351.4)
Total debt issuance costs	(7.0)		(12.8)
Junior Convertible Debt | 2017 Junior Convertible Debt
Debt Instrument [Line Items]
Total unamortized discount	(337.4)		(347.2)
Total debt issuance costs	$ (3.1)		$ (3.2)