Fair Value of Financial Instruments (Schedule of carrying amounts and fair values) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Mar. 31, 2016
Carrying Value | Senior Subordinated Convertible Debenture Due 2025
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Disclosure for Subordinated Convertible Debentures	$ 1,238,731	$ 1,216,313
Carrying Value | Junior Subordinated Convertible Debentures Due 2037
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Disclosure for Subordinated Convertible Debentures	196,868	193,936
Estimate of Fair Value Measurement | Senior Subordinated Convertible Debenture Due 2025 | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Disclosure for Subordinated Convertible Debentures	2,216,539	1,762,088
Estimate of Fair Value Measurement | Junior Subordinated Convertible Debentures Due 2037 | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Disclosure for Subordinated Convertible Debentures	$ 1,484,805	$ 1,143,117