Fair Value of Financial Instruments (Schedule of carrying amounts and fair values) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016
Senior Subordinated Convertible Debenture Due 2025 [Member] | Estimate of Fair Value [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Disclosure for Subordinated Convertible Debentures	$ 1,919,356	$ 1,762,088
Senior Subordinated Convertible Debenture Due 2025 [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Disclosure for Subordinated Convertible Debentures	1,227,444	1,216,313
Junior Subordinated Convertible Debentures Due 2037 | Estimate of Fair Value [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Disclosure for Subordinated Convertible Debentures	1,187,375	1,143,117
Junior Subordinated Convertible Debentures Due 2037 | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Disclosure for Subordinated Convertible Debentures	$ 195,315	$ 193,936