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v3.20.4

Receivables (Tables) | 12 Months Ended
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Dec. 31, 2020
Transfers and Servicing [Abstract] |
Schedule of Allowance for Doubtful Accounts | The following table details the changes in the allowance for doubtful accounts related to customer receivables and other receivables:
| Customer receivables |  | Other receivables
| 2020 |  | 2019 |  | 2018 |  | 2020 |  | 2019 |  | 2018
Balance at beginning of year | $ | 1 |  |  | $ | 1 |  |  | $ | 3 |  |  | $ | 15 |  |  | $ | 15 |  |  | $ | 15 |
Provision for doubtful accounts | 1 |  |  | 2 |  |  | — |  |  | 3 |  |  | 7 |  |  | 2 |
Write off of uncollectible accounts | — |  |  | (1) |  |  | (2) |  |  | (1) |  |  | (2) |  |  | (1) |
Recoveries of prior write-offs | — |  |  | — |  |  | — |  |  | (1) |  |  | (3) |  |  | (3) |
Other | (1) |  |  | (1) |  |  | — |  |  | 3 |  |  | (2) |  |  | 2 |
Balance at end of year | $ | 1 |  |  | $ | 1 |  |  | $ | 1 |  |  | $ | 19 |  |  | $ | 15 |  |  | $ | 15 |

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