Fair Value Measurements (Carrying Amount and Estimated Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020
Carrying [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 167,477	$ 410,405
Accounts receivable	626,991	740,105
Investment in business purchase option		146,500
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	167,477	410,405
Accounts receivable	626,991	740,105
Investment in business purchase option		146,500
Significant Other Observable Inputs (Level 2) | Carrying [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt component of tangible equity units	28,058	35,431
Significant Other Observable Inputs (Level 2) | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt component of tangible equity units	28,624	34,806
Senior Notes due March 1, 2025 [Member] | Significant Other Observable Inputs (Level 2) | Carrying [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	1,317,305	997,772
Senior Notes due March 1, 2025 [Member] | Significant Other Observable Inputs (Level 2) | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	1,315,063	950,000
Senior Credit Facilities [Member] | Significant Other Observable Inputs (Level 2) | Carrying [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	3,648,824	3,682,457
Senior Credit Facilities [Member] | Significant Other Observable Inputs (Level 2) | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 3,678,387	$ 3,452,687