Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of revenue
Revenue from contracts with customers	$ 954	$ 957	$ 1,899	$ 1,907
Operating lease income	19	16	38	32
Total operating revenues	973	973	1,937	1,939
Transferred over time
Disaggregation of revenue
Revenue from contracts with customers	734	741	1,477	1,466
Transferred over time | Retail service
Disaggregation of revenue
Revenue from contracts with customers	658	662	1,329	1,322
Transferred over time | Inbound roaming
Disaggregation of revenue
Revenue from contracts with customers	41	44	77	78
Transferred over time | Other service
Disaggregation of revenue
Revenue from contracts with customers	35	35	71	66
Transferred at point in time | Equipment sales
Disaggregation of revenue
Revenue from contracts with customers	$ 220	$ 216	$ 422	$ 441