Revenue Recognition - Disaggregation Of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of revenue
Revenue from contracts with customers	$ 1,008	$ 985	$ 2,915	$ 2,867
Transferred over time
Disaggregation of revenue
Revenue from contracts with customers	751	743	2,217	2,175
Transferred over time | Retail service
Disaggregation of revenue
Revenue from contracts with customers	663	659	1,984	1,960
Transferred over time | Inbound roaming
Disaggregation of revenue
Revenue from contracts with customers	54	50	132	116
Transferred over time | Other service
Disaggregation of revenue
Revenue from contracts with customers	34	34	101	99
Transferred at point in time | Equipment sales
Disaggregation of revenue
Revenue from contracts with customers	$ 257	$ 242	$ 698	$ 692