Segment and Geographic Information - Segment Information (Details)	12 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reporting segments | segment	2
Revenues	$ 19,577,000,000	$ 20,753,000,000	$ 21,733,000,000
Segment Profit	2,061,000,000	3,269,000,000	2,989,000,000
Depreciation and amortization	1,359,000,000	1,429,000,000	1,269,000,000
Amortization of acquired intangible assets	583,000,000	539,000,000	526,000,000
Reconciliation of Consolidated Operating Income to Income Before Taxes [Abstract]
Segment Profit	2,061,000,000	3,269,000,000	2,989,000,000
Interest income	165,000,000	128,000,000	89,000,000
Interest expense	(383,000,000)	(334,000,000)	(320,000,000)
Restructuring costs	(252,000,000)	(465,000,000)	(789,000,000)
Transaction, separation, and integration-related costs	(318,000,000)	(401,000,000)	(359,000,000)
Amortization of acquired intangible assets	(583,000,000)	(539,000,000)	(526,000,000)
Goodwill impairment losses	(6,794,000,000)	0	0	[1]
Gain on arbitration award	632,000,000	0	0
Pension and OPEB actuarial and settlement gains (losses)	244,000,000	(143,000,000)	220,000,000
(Loss) income from continuing operations, before taxes	(5,228,000,000)	1,515,000,000	1,304,000,000
GBS
Reconciliation of Consolidated Operating Income to Income Before Taxes [Abstract]
Goodwill impairment losses	(3,789,000,000)
GIS
Reconciliation of Consolidated Operating Income to Income Before Taxes [Abstract]
Goodwill impairment losses	(3,005,000,000)
Operating segments
Segment Reporting Information [Line Items]
Revenues	19,577,000,000	20,753,000,000	21,733,000,000
Segment Profit	2,308,000,000	3,556,000,000	3,168,000,000
Depreciation and amortization	1,250,000,000	1,302,000,000	1,177,000,000
Reconciliation of Consolidated Operating Income to Income Before Taxes [Abstract]
Segment Profit	2,308,000,000	3,556,000,000	3,168,000,000
Operating segments | GBS
Segment Reporting Information [Line Items]
Revenues	9,111,000,000	8,684,000,000	9,254,000,000
Segment Profit	1,301,000,000	1,645,000,000	1,525,000,000
Depreciation and amortization	199,000,000	90,000,000	99,000,000
Reconciliation of Consolidated Operating Income to Income Before Taxes [Abstract]
Segment Profit	1,301,000,000	1,645,000,000	1,525,000,000
Operating segments | GIS
Segment Reporting Information [Line Items]
Revenues	10,466,000,000	12,069,000,000	12,479,000,000
Segment Profit	1,007,000,000	1,911,000,000	1,643,000,000
Depreciation and amortization	1,051,000,000	1,212,000,000	1,078,000,000
Reconciliation of Consolidated Operating Income to Income Before Taxes [Abstract]
Segment Profit	1,007,000,000	1,911,000,000	1,643,000,000
All Other
Segment Reporting Information [Line Items]
Revenues	0	0	0
Segment Profit	(247,000,000)	(287,000,000)	(179,000,000)
Depreciation and amortization	109,000,000	127,000,000	92,000,000
Reconciliation of Consolidated Operating Income to Income Before Taxes [Abstract]
Segment Profit	$ (247,000,000)	$ (287,000,000)	$ (179,000,000)

[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.