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Discontinued Operations - Components of Discontinued Operations from Merger Date (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income from discontinued operations	$ 320	$ 0	$ 320	$ 0
Net cash provided by operating activities from the Prepaid Business	611		$ 611
Discontinued Operations, Disposed of by Sale \| Prepaid Transaction
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Service revenues	1,270
Selling, general and administrative	314
Total operating expenses	838
Pretax income from discontinued operations	432
Income tax expense	(112)
Income from discontinued operations	320
Discontinued Operations, Disposed of by Sale \| Prepaid Transaction \| Prepaid revenues
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Service revenues	973
Discontinued Operations, Disposed of by Sale \| Prepaid Transaction \| Roaming and other services revenue
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Service revenues	27
Discontinued Operations, Disposed of by Sale \| Prepaid Transaction \| Service
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Service revenues	1,000
Cost of services and sales	25
Discontinued Operations, Disposed of by Sale \| Prepaid Transaction \| Equipment revenues
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Service revenues	270
Cost of services and sales	$ 499