Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized debt issuance expense	$ 34.2	$ 38.3
Fair Value | 2017 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	2,459.2	2,106.2
Fair Value | 2015 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	3,079.1	2,481.7
Fair Value | 2017 Junior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	876.9	586.6
Fair Value | 2007 Junior Subordinated Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	0.0	445.1
Carrying Amount | 2017 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	1,437.6	1,384.9
Carrying Amount | 2015 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	1,309.9	1,261.8
Carrying Amount | 2017 Junior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	326.7	262.3
Carrying Amount | 2007 Junior Subordinated Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	$ 0.0	$ 50.0