Allowance for Uncollectible Accounts - Schedule of Allowances for Uncollectible Accounts (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Accounts Receivable, Allowance for Credit Loss [Roll Forward] | |||
| Balance as of January 1 | $ (45) | $ (42) | $ (40) |
| Amounts charged to expense | (28) | (33) | (29) |
| Amounts written off | 32 | 34 | 30 |
| Recoveries of amounts written off | 0 | (4) | (3) |
| Balance as of December 31 | $ (41) | $ (45) | $ (42) |