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Fair Value Measurements (Details) - Recurring - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Assets |  |
Fair value, assets | $ 1,349 | $ 1,693
Liabilities |  |
Fair value, liabilities | (70) | (83)
Net assets (liabilities) | 1,279 | 1,610
Commodity contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | (4)
FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | (5) | 0
NUG contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | (65) | (79)
Corporate debt securities |  |
Assets |  |
Fair value, assets | 474 | 476
FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 5 | 3
Equity securities |  |
Assets |  |
Fair value, assets | 298 | 297
Foreign government debt securities |  |
Assets |  |
Fair value, assets | 16 | 23
U.S. government debt securities |  |
Assets |  |
Fair value, assets | 19 | 21
U.S. state debt securities |  |
Assets |  |
Fair value, assets | 248 | 247
Other |  |
Assets |  |
Fair value, assets | 289 | 626
Level 1 |  |
Assets |  |
Fair value, assets | 554 | 885
Liabilities |  |
Fair value, liabilities | 0 | 0
Net assets (liabilities) | 554 | 885
Level 1 | Commodity contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
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 | 298 | 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 | 256 | 588
Level 2 |  |
Assets |  |
Fair value, assets | 790 | 805
Liabilities |  |
Fair value, liabilities | 0 | (4)
Net assets (liabilities) | 790 | 801
Level 2 | Commodity contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | (4)
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 | 474 | 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 | 16 | 23
Level 2 | U.S. government debt securities |  |
Assets |  |
Fair value, assets | 19 | 21
Level 2 | U.S. state debt securities |  |
Assets |  |
Fair value, assets | 248 | 247
Level 2 | Other |  |
Assets |  |
Fair value, assets | 33 | 38
Level 3 |  |
Assets |  |
Fair value, assets | 5 | 3
Liabilities |  |
Fair value, liabilities | (70) | (79)
Net assets (liabilities) | (65) | (76)
Level 3 | Commodity contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | 0 | 0
Level 3 | FTRs | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | (5) | 0
Level 3 | NUG contracts | Derivative Liabilities |  |
Liabilities |  |
Fair value, liabilities | (65) | (79)
Level 3 | Corporate debt securities |  |
Assets |  |
Fair value, assets | 0 | 0
Level 3 | FTRs | Derivative Assets |  |
Assets |  |
Fair value, assets | 5 | 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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