Fair Value - Financial Instruments (Details) - USD ($)	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	[1]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, net of current maturities	$ 8,672,000,000	$ 5,470,000,000
Impairment of goodwill	6,794,000,000	0	$ 0
Impairment of tangible assets	0	0
Impairment of intangible assets	0	0
Impairment of long-lived assets	0	0
Fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, net of current maturities	8,200,000,000	5,600,000,000
Carrying value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, net of current maturities	$ 8,400,000,000	$ 5,600,000,000

[1] As a result of the USPS Separation, the Consolidated Statements of Operations, Consolidated Balance Sheets, and related financial information reflect USPS's operations and assets and liabilities as discontinued operations for all periods presented. The cash flows of USPS have not been segregated and are included in the Consolidated Statement of Cash flows for the fiscal year ended March 31, 2018 and through the separation date of May 31, 2018 in the Consolidated Statement of Cash Flows for the fiscal year ended March 31, 2019.