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Fair Value of Financial Instruments - Assets and Liabilities Measured and Recorded at Fair Value (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in securities (classified within other current and non-current assets) | $ 25 | $ 20
Other trust fund investments: |  |
Other trust fund investments | 1 | 1
Measured using net asset value practical expedient: |  |
Total assets | 1,745 | 1,993
Derivative liabilities: |  |
Total liabilities | 884 | 1,103
Commodity contracts |  |
Derivative assets: |  |
Derivative assets | 821 | 1,170
Derivative liabilities: |  |
Commodity contracts | 884 | 1,103
Cash and cash equivalents |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 23 | 17
U.S. government and federal agency obligations |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 70 | 68
Federal agency mortgage-backed securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 89 | 100
Commercial mortgage-backed securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 36 | 29
Corporate debt securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 144 | 109
Equity securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 434 | 388
Measured using net asset value practical expedient: |  |
Equity contracts | 8 | 8
Foreign government fixed income securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 7 | 5
Equity securities-nuclear trust fund investments |  |
Measured using net asset value practical expedient: |  |
Equity contracts | 87 | 78
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in securities (classified within other current and non-current assets) | 10 | 0
Other trust fund investments: |  |
Other trust fund investments | 1 | 1
Measured using net asset value practical expedient: |  |
Total assets | 597 | 558
Derivative liabilities: |  |
Total liabilities | 86 | 143
Level 1 | Commodity contracts |  |
Derivative assets: |  |
Derivative assets | 59 | 84
Derivative liabilities: |  |
Commodity contracts | 86 | 143
Level 1 | Cash and cash equivalents |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 23 | 17
Level 1 | U.S. government and federal agency obligations |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 69 | 68
Level 1 | Federal agency mortgage-backed securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Level 1 | Commercial mortgage-backed securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Level 1 | Corporate debt securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Level 1 | Equity securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 434 | 388
Measured using net asset value practical expedient: |  |
Equity contracts | 0 | 0
Level 1 | Foreign government fixed income securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 1 | 0
Level 1 | Equity securities-nuclear trust fund investments |  |
Measured using net asset value practical expedient: |  |
Equity contracts | 0 | 0
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in securities (classified within other current and non-current assets) | 15 | 20
Other trust fund investments: |  |
Other trust fund investments | 0 | 0
Measured using net asset value practical expedient: |  |
Total assets | 914 | 1,156
Derivative liabilities: |  |
Total liabilities | 643 | 805
Level 2 | Commodity contracts |  |
Derivative assets: |  |
Derivative assets | 623 | 893
Derivative liabilities: |  |
Commodity contracts | 643 | 805
Level 2 | Cash and cash equivalents |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Level 2 | U.S. government and federal agency obligations |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 1 | 0
Level 2 | Federal agency mortgage-backed securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 89 | 100
Level 2 | Commercial mortgage-backed securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 36 | 29
Level 2 | Corporate debt securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 144 | 109
Level 2 | Equity securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Measured using net asset value practical expedient: |  |
Equity contracts | 0 | 0
Level 2 | Foreign government fixed income securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 6 | 5
Level 2 | Equity securities-nuclear trust fund investments |  |
Measured using net asset value practical expedient: |  |
Equity contracts | 0 | 0
Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in securities (classified within other current and non-current assets) | 0 | 0
Other trust fund investments: |  |
Other trust fund investments | 0 | 0
Measured using net asset value practical expedient: |  |
Total assets | 139 | 193
Derivative liabilities: |  |
Total liabilities | 155 | 155
Level 3 | Commodity contracts |  |
Derivative assets: |  |
Derivative assets | 139 | 193
Derivative liabilities: |  |
Commodity contracts | 155 | 155
Level 3 | Cash and cash equivalents |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Level 3 | U.S. government and federal agency obligations |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Level 3 | Federal agency mortgage-backed securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Level 3 | Commercial mortgage-backed securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Level 3 | Corporate debt securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Level 3 | Equity securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Measured using net asset value practical expedient: |  |
Equity contracts | 0 | 0
Level 3 | Foreign government fixed income securities |  |
Nuclear trust fund investments: |  |
Nuclear trust fund investments | 0 | 0
Level 3 | Equity securities-nuclear trust fund investments |  |
Measured using net asset value practical expedient: |  |
Equity contracts | $ 0 | $ 0

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