Summary of Significant Accounting Policies - Narrative (Details) $ in Millions			3 Months Ended	6 Months Ended
	May 31, 2019	Mar. 31, 2019	Sep. 30, 2019 USD ($) country	Sep. 30, 2019 USD ($) client country
Accounting Policies [Abstract]
Number of clients served | client				6,000
Number of countries in which entity operates | country			70	70
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Change in depreciation expense | $			$ (56)	$ (111)
USPS Separation | Discontinued Operations, Disposed of by Means Other than Sale, Spinoff
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Share exchange ratio	0.5
Minimum | Computers and related equipment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Property, plant and equipment useful life (in years)		4 years		4 years
Maximum | Computers and related equipment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Property, plant and equipment useful life (in years)		5 years		7 years