Debt and Credit Facility - Components (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	May 31, 2018
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	$ (1,238.9)	$ (1,268.7)	$ (10.5)
Debt issuance cost	(77.6)	(83.6)	$ (24.4)
Revolving Credit Facility
Debt Instrument [Line Items]
Debt issuance cost	(13.6)	(14.7)
Term Loan Facility
Debt Instrument [Line Items]
Debt issuance cost	(16.8)	(19.4)
2023 Senior Secured Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(4.2)	(4.4)
Debt issuance cost	(9.6)	(10.2)
2021 Senior Secured Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(3.4)	(3.8)
Debt issuance cost	(7.8)	(8.8)
2017 Senior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(547.8)	(561.9)
Debt issuance cost	(14.2)	(14.5)
2015 Senior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(338.7)	(351.4)
Debt issuance cost	(12.4)	(12.8)
2017 Junior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(344.8)	(347.2)
Debt issuance cost	$ (3.2)	$ (3.2)