Fair Value of Financial Instruments (Schedule of carrying amounts and fair values) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018
Carrying Value | 2023 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	$ 982.8	$ 0.0
Carrying Value | 2021 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	983.1	0.0
Carrying Value | Term Loan Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	2,965.8	0.0
Carrying Value | 2017 Senior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,451.3	1,437.6
Carrying Value | 2015 Senior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,322.3	1,309.9
Carrying Value | 2017 Junior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	328.9	326.7
Estimate of Fair Value Measurement | 2023 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,005.5	0.0
Estimate of Fair Value Measurement | 2021 Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,001.7	0.0
Estimate of Fair Value Measurement | Term Loan Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	3,000.0	0.0
Estimate of Fair Value Measurement | 2017 Senior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	2,416.7	2,459.2
Estimate of Fair Value Measurement | 2015 Senior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	3,011.2	3,079.1
Estimate of Fair Value Measurement | 2017 Junior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	$ 814.2	$ 876.9