Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Mar. 31, 2019
Carrying Amount | 2023 Senior Secured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	$ 987.1	$ 985.4
Carrying Amount | 2021 Senior Secured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	990.2	987.4
Carrying Amount | Term Loan Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,707.5	1,892.1
Carrying Amount | 2017 Senior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,522.8	1,493.6
Carrying Amount | 2015 Senior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,387.3	1,360.8
Carrying Amount | 2017 Junior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	340.8	335.9
Fair Value | 2023 Senior Secured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,055.0	1,020.1
Fair Value | 2021 Senior Secured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,022.5	1,008.1
Fair Value | Term Loan Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,723.5	1,911.5
Fair Value | 2017 Senior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	2,666.5	2,285.4
Fair Value | 2015 Senior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	3,270.2	2,810.6
Fair Value | 2017 Junior Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	$ 888.6	$ 740.8