Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of revenue
Revenue from contracts with customers	$ 1,007	$ 1,008	$ 2,907	$ 2,915
Operating lease income	20	23	57	55
Total operating revenues	1,027	1,031	2,964	2,970
Transferred over time
Disaggregation of revenue
Revenue from contracts with customers	755	751	2,233	2,217
Transferred over time | Retail service
Disaggregation of revenue
Revenue from contracts with customers	674	663	2,004	1,984
Transferred over time | Inbound roaming
Disaggregation of revenue
Revenue from contracts with customers	42	54	119	132
Transferred over time | Other service
Disaggregation of revenue
Revenue from contracts with customers	39	34	110	101
Transferred at point in time | Equipment sales
Disaggregation of revenue
Revenue from contracts with customers	$ 252	$ 257	$ 674	$ 698