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FAIR VALUE MEASUREMENTS - Recurring Assets and Liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Liabilities |  |
Investment excludes receivables, payables and accrued income | $ 1 | $ (16)
Recurring |  |
Assets |  |
Fair value, assets | 2,056 | 1,828
Liabilities |  |
Fair value, liabilities | 0 | (17)
Net assets (liabilities) | 2,056 | 1,811
Recurring | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | (1)
Recurring | NUG contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | (16)
Recurring | Corporate debt securities |  |
Assets |  |
Fair value, assets | 0 | 135
Recurring | FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 3 | 4
Recurring | Equity securities |  |
Assets |  |
Fair value, assets | 2 | 2
Recurring | U.S. state debt securities |  |
Assets |  |
Fair value, assets | 276 | 271
Recurring | Other |  |
Assets |  |
Fair value, assets | 1,775 | 1,416
Recurring | Level 1 |  |
Assets |  |
Fair value, assets | 1,736 | 629
Liabilities |  |
Fair value, liabilities | 0 | 0
Net assets (liabilities) | 1,736 | 629
Recurring | Level 1 | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Recurring | Level 1 | NUG contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Recurring | Level 1 | Corporate debt securities |  |
Assets |  |
Fair value, assets | 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 | Other |  |
Assets |  |
Fair value, assets | 1,734 | 627
Recurring | Level 2 |  |
Assets |  |
Fair value, assets | 317 | 1,195
Liabilities |  |
Fair value, liabilities | 0 | 0
Net assets (liabilities) | 317 | 1,195
Recurring | Level 2 | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Recurring | Level 2 | NUG contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Recurring | Level 2 | Corporate debt securities |  |
Assets |  |
Fair value, assets | 0 | 135
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 | 276 | 271
Recurring | Level 2 | Other |  |
Assets |  |
Fair value, assets | 41 | 789
Recurring | Level 3 |  |
Assets |  |
Fair value, assets | 3 | 4
Liabilities |  |
Fair value, liabilities | 0 | (17)
Net assets (liabilities) | 3 | (13)
Recurring | Level 3 | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | (1)
Recurring | Level 3 | NUG contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | (16)
Recurring | Level 3 | Corporate debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Recurring | Level 3 | FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 3 | 4
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 | Other |  |
Assets |  |
Fair value, assets | $ 0 | $ 0

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