Debt and Credit Facility Components (Details) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2017	Feb. 28, 2017	Feb. 28, 2015
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	$ (1,399.8)	$ (1,516.5)
Debt issuance costs	(41.9)	(46.8)	$ (33.7)
December 2015 Amended Credit Facility with Maturity Dates of June 2018 and February 2020
Debt Instrument [Line Items]
Debt issuance costs	(6.7)	(8.5)
2017 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(629.3)	(667.5)
Debt issuance costs	(16.5)	(17.6)
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(412.1)	(446.6)
Debt issuance costs	(15.3)	(16.6)
2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(358.4)	(309.3)
Debt issuance costs	(3.4)	(3.4)		$ (30.3)
2007 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	0.0	(93.1)
Debt issuance costs	$ 0.0	$ (0.7)