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Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2020
Accounting Policies [Abstract] |
Restrictions on Cash and Cash Equivalents | The following table presents a reconciliation of cash and cash equivalents and restricted cash reported on our Consolidated Balance Sheets that sum to the total of the amount shown on our Consolidated Statements of Cash Flows (in millions):
| December 31,
| 2020 |  | 2019
Cash and cash equivalents | $ | 25 |  |  | $ | 47 |
Restricted cash | 38 |  |  | 37 |
Total cash and cash equivalents and restricted cash | $ | 63 |  |  | $ | 84 |
Schedule of Change in Asset Retirement Obligation | The following table presents the change in the liability for asset retirement obligations, substantially all of which is reflected in “Other long-term liabilities and deferred credits” on our Consolidated Balance Sheets as of December 31, 2020, 2019 and 2018 (in millions):
| December 31,
| 2020 |  | 2019 |  | 2018
Beginning balance | $ | 137 |  |  | $ | 109 |  |  | $ | 103 |
Liabilities incurred | 12 |  |  | 3 |  |  | 3 |
Liabilities settled | (1) |  |  | (3) |  |  | (3) |
Accretion expense | 5 |  |  | 5 |  |  | 4 |
Revisions in estimated cash flows | (18) |  |  | 23 |  |  | 2 |
Ending balance | $ | 135 |  |  | $ | 137 |  |  | $ | 109 |

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