Fair Value of Financial Instruments (Schedule of carrying amounts and fair values) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017
Carrying Value | 2017 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	$ 1,424,169	$ 1,384,914
Carrying Value | 2015 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	1,297,605	1,261,787
Carrying Value | 2017 Junior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	324,528	262,298
Carrying Value | 2007 Junior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	0	49,952
Estimate of Fair Value Measurement | 2017 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	2,436,742	2,106,225
Estimate of Fair Value Measurement | 2015 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	2,952,993	2,481,708
Estimate of Fair Value Measurement | 2017 Junior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	814,329	586,609
Estimate of Fair Value Measurement | 2007 Junior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for subordinated convertible debentures	$ 0	$ 445,142