Revenue from Contracts with Customers - Remaining Performance Obligations, Branded Postpaid Contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Apr. 01, 2020
Postpaid service revenues
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation	$ 1,500
Remaining contract duration (in years)	24 months
Promotional bill credits \| Sprint
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation		$ 1,000
Service performance obligations \| Sprint
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation	$ 2,100	4,800
Lease performance obligation \| Sprint
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation	$ 1,200	$ 2,600
Remaining contract duration (in years)	18 months
Minimum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining contract duration (in years)	1 year
Maximum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining contract duration (in years)	9 years