Summary of Significant Accounting Policies - Narrative (Details) $ in Millions			3 Months Ended	9 Months Ended
	Mar. 31, 2019	May 31, 2018	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Change in depreciation expense			$ (68)	$ (179)
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