Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 11,878	$ 11,061	$ 10,979	$ 11,080	$ 11,445	$ 10,839	$ 10,571	$ 10,455	$ 44,998	$ 43,310	$ 40,604
Branded postpaid phone revenues
Disaggregation of Revenue [Line Items]
Revenues									21,329	19,745	18,371
Branded postpaid other revenues
Disaggregation of Revenue [Line Items]
Revenues									1,344	1,117	1,077
Branded postpaid service revenues
Disaggregation of Revenue [Line Items]
Revenues									22,673	20,862	19,448
Equipment revenues from the lease of mobile communication devices
Disaggregation of Revenue [Line Items]
Revenues									$ 599	$ 692	$ 877