Debt and Credit Facility Components (Details) - USD ($) $ in Millions	Jun. 30, 2018	May 31, 2018	Mar. 31, 2018	Feb. 28, 2017	Feb. 28, 2015
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	$ (1,355.7)	$ (10.5)	$ (1,372.9)
Debt issuance costs	(110.0)	$ (24.4)	(40.1)	$ (33.7)
2023 Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(5.2)		0.0
Debt issuance costs	(12.0)		0.0
2021 Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(5.1)		0.0
Debt issuance costs	(11.8)		0.0
December 2015 Amended Credit Facility with Maturity Dates of June 2018 and February 2020
Debt Instrument [Line Items]
Debt issuance costs	(18.6)		(5.9)
Term Loan Facility
Debt Instrument [Line Items]
Debt issuance costs	(34.2)		0.0
2017 Senior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(602.9)		(616.3)
Debt issuance costs	(15.8)		(16.1)
2015 Senior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(388.4)		(400.3)
Debt issuance costs	(14.3)		(14.8)
2017 Junior Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(354.1)		(356.3)
Debt issuance costs	$ (3.3)		$ (3.3)		$ (30.3)