Additional Financial Information - Related Party Transactions (Details) - Affiliates - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2020 |
Jun. 30, 2019 |
Jun. 30, 2020 |
Jun. 30, 2019 |
|
| Related Party Transaction [Line Items] | ||||
| Discount related to roaming expenses | $ (2) | $ 0 | $ (5) | $ (2) |
| Fees incurred for use of the T-Mobile brand | 20 | 23 | 43 | 44 |
| International long distance agreement | 11 | 11 | 22 | 20 |
| Reimbursement of certain administrative expenses | $ 1 | $ 3 | $ 3 | $ 5 |