Revenue from Contracts with Customers - Remaining Performance Obligations, Branded Postpaid Contracts (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Branded postpaid contracts
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation	$ 234
Remaining contract duration	24 months
Minimum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining contract duration	1 year
Maximum
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining contract duration	11 years