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Fair Value Measurements (Details) - Recurring - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Assets |  |
Fair value, assets | $ 1,571 | $ 1,693
Liabilities |  |
Fair value, liabilities | (53) | (79)
Net assets (liabilities) | 1,518 | 1,614
FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | (1) | 0
NUG contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | (52) | (79)
Corporate debt securities |  |
Assets |  |
Fair value, assets | 405 | 476
FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 10 | 3
Equity securities |  |
Assets |  |
Fair value, assets | 329 | 297
Foreign government debt securities |  |
Assets |  |
Fair value, assets | 12 | 23
U.S. government debt securities |  |
Assets |  |
Fair value, assets | 22 | 21
U.S. state debt securities |  |
Assets |  |
Fair value, assets | 251 | 247
Other |  |
Assets |  |
Fair value, assets | 542 | 626
Level 1 |  |
Assets |  |
Fair value, assets | 765 | 885
Liabilities |  |
Fair value, liabilities | 0 | 0
Net assets (liabilities) | 765 | 885
Level 1 | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Level 1 | NUG contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Level 1 | Corporate debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 1 | FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 0 | 0
Level 1 | Equity securities |  |
Assets |  |
Fair value, assets | 329 | 297
Level 1 | Foreign government debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 1 | U.S. government debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 1 | U.S. state debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 1 | Other |  |
Assets |  |
Fair value, assets | 436 | 588
Level 2 |  |
Assets |  |
Fair value, assets | 796 | 805
Liabilities |  |
Fair value, liabilities | 0 | 0
Net assets (liabilities) | 796 | 805
Level 2 | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Level 2 | NUG contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Level 2 | Corporate debt securities |  |
Assets |  |
Fair value, assets | 405 | 476
Level 2 | FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 0 | 0
Level 2 | Equity securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 2 | Foreign government debt securities |  |
Assets |  |
Fair value, assets | 12 | 23
Level 2 | U.S. government debt securities |  |
Assets |  |
Fair value, assets | 22 | 21
Level 2 | U.S. state debt securities |  |
Assets |  |
Fair value, assets | 251 | 247
Level 2 | Other |  |
Assets |  |
Fair value, assets | 106 | 38
Level 3 |  |
Assets |  |
Fair value, assets | 10 | 3
Liabilities |  |
Fair value, liabilities | (53) | (79)
Net assets (liabilities) | (43) | (76)
Level 3 | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | (1) | 0
Level 3 | NUG contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | (52) | (79)
Level 3 | Corporate debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 3 | FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 10 | 3
Level 3 | Equity securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 3 | Foreign government debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 3 | U.S. government debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 3 | U.S. state debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 3 | Other |  |
Assets |  |
Fair value, assets | $ 0 | $ 0

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