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Fair Value Measurements - Recurring Assets and Liabilities (Details) - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Liabilities |  |
Investment excludes receivables, payables and accrued income |  | $ 1
Recurring |  |
Assets |  |
Fair value, assets | $ 1,633 | 2,123
Liabilities |  |
Fair value, liabilities | (2) | 0
Net assets (liabilities) | 1,631 | 2,123
Recurring | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | (2) | 0
Recurring | FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 5 | 3
Recurring | Equity securities |  |
Assets |  |
Fair value, assets | 2 | 2
Recurring | U.S. state debt securities |  |
Assets |  |
Fair value, assets | 269 | 276
Recurring | Cash, cash equivalents and restricted cash |  |
Assets |  |
Fair value, assets | 1,312 | 1,801
Recurring | Other |  |
Assets |  |
Fair value, assets | 45 | 41
Recurring | Level 1 |  |
Assets |  |
Fair value, assets | 1,314 | 1,803
Liabilities |  |
Fair value, liabilities | 0 | 0
Net assets (liabilities) | 1,314 | 1,803
Recurring | Level 1 | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Recurring | Level 1 | FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 0 | 0
Recurring | Level 1 | Equity securities |  |
Assets |  |
Fair value, assets | 2 | 2
Recurring | Level 1 | U.S. state debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Recurring | Level 1 | Cash, cash equivalents and restricted cash |  |
Assets |  |
Fair value, assets | 1,312 | 1,801
Recurring | Level 1 | Other |  |
Assets |  |
Fair value, assets | 0 | 0
Recurring | Level 2 |  |
Assets |  |
Fair value, assets | 314 | 317
Liabilities |  |
Fair value, liabilities | 0 | 0
Net assets (liabilities) | 314 | 317
Recurring | Level 2 | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Recurring | Level 2 | FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 0 | 0
Recurring | Level 2 | Equity securities |  |
Assets |  |
Fair value, assets | 0 | 0
Recurring | Level 2 | U.S. state debt securities |  |
Assets |  |
Fair value, assets | 269 | 276
Recurring | Level 2 | Cash, cash equivalents and restricted cash |  |
Assets |  |
Fair value, assets | 0 | 0
Recurring | Level 2 | Other |  |
Assets |  |
Fair value, assets | 45 | 41
Recurring | Level 3 |  |
Assets |  |
Fair value, assets | 5 | 3
Liabilities |  |
Fair value, liabilities | (2) | 0
Net assets (liabilities) | 3 | 3
Recurring | Level 3 | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | (2) | 0
Recurring | Level 3 | FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 5 | 3
Recurring | Level 3 | Equity securities |  |
Assets |  |
Fair value, assets | 0 | 0
Recurring | Level 3 | U.S. state debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Recurring | Level 3 | Cash, cash equivalents and restricted cash |  |
Assets |  |
Fair value, assets | 0 | 0
Recurring | Level 3 | Other |  |
Assets |  |
Fair value, assets | $ 0 | $ 0

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