Income Taxes - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2020	Apr. 01, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Indirect tax effects excluded	$ 144
Federal and state tax credits	411		$ 331
Uncertain tax benefit reserves	1,159		514	$ 462	$ 412
Valuation allowance	(878)		(129)	(210)
Unrecognized tax benefits that would impact effective tax rate	857		$ 310	$ 263
Sprint
Income Tax Contingency [Line Items]
Deferred tax assets		$ 848
Federal
Income Tax Contingency [Line Items]
Unrecognized tax benefits, net operating loss	3,500
Operating loss carryforwards	176
Federal | Research Tax Credit Carryforward
Income Tax Contingency [Line Items]
Federal and state tax credits	569
State
Income Tax Contingency [Line Items]
Unrecognized tax benefits, net operating loss	1,500
Operating loss carryforwards	455
Foreign Tax Authority
Income Tax Contingency [Line Items]
Unrecognized tax benefits, net operating loss	67
Operating loss carryforwards	$ 26