Debt and Credit Facility - Components (Details) - USD ($) $ in Millions	Mar. 31, 2019	May 31, 2018	Mar. 31, 2018	Feb. 28, 2017	Feb. 28, 2015
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	$ (1,268.7)	$ (10.5)	$ (1,372.9)
Debt issuance cost	(83.6)	$ (24.4)	(40.1)	$ (33.7)
Revolving Credit Facility
Debt Instrument [Line Items]
Debt issuance cost	(14.7)		(5.9)
Term Loan Facility
Debt Instrument [Line Items]
Debt issuance cost	(19.4)
2023 Senior Secured Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(4.4)
Debt issuance cost	(10.2)
2021 Senior Secured Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(3.8)
Debt issuance cost	(8.8)
2017 Senior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(561.9)		(616.3)
Debt issuance cost	(14.5)		(16.1)
2015 Senior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(351.4)		(400.3)
Debt issuance cost	(12.8)		(14.8)		$ (30.3)
2017 Junior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(347.2)		(356.3)
Debt issuance cost	$ (3.2)		$ (3.3)