Debt and Credit Facility - Components (Details) - USD ($) $ in Millions	Dec. 31, 2018	May 31, 2018	Mar. 31, 2018	Feb. 28, 2017	Feb. 28, 2015
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	$ (1,298.0)	$ (10.5)	$ (1,372.9)
Debt issuance costs	(96.5)	$ (24.4)	(40.1)	$ (33.7)
Revolving Credit Facility
Debt Instrument [Line Items]
Debt issuance costs	(15.6)		(5.9)
Term Loan Facility
Debt Instrument [Line Items]
Debt issuance costs	(28.7)		0.0
2023 Senior Secured Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(4.7)		0.0
Debt issuance costs	(10.8)		0.0
2021 Senior Secured Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(4.3)		0.0
Debt issuance costs	(9.8)		0.0
2017 Senior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(575.7)		(616.3)
Debt issuance costs	(15.0)		(16.1)
2015 Senior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(363.9)		(400.3)
Debt issuance costs	(13.3)		(14.8)
2017 Junior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(349.4)		(356.3)
Debt issuance costs	$ (3.3)		$ (3.3)		$ (30.3)