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Fair Value of Financial Instruments - Assets and Liabilities Measured and Recorded at Fair Value (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | $ 798 | $ 1,248
Derivative liabilities | 752 | 1,376
Commodity contracts |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | 745 | 1,199
Derivative liabilities | 693 | 1,288
Interest rate contracts |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | 53 | 49
Derivative liabilities | 59 | 88
Fair Value, Measurements, Recurring |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Debt securities | 19 | 17
Available-for-sale Securities | 3 | 10
U.S. government and federal agency obligations | 1 | 1
Total assets | 1,513 | 1,886
Total liabilities | 752 | 1,376
Fair Value, Measurements, Recurring | Commodity contracts |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | 745 | 1,199
Derivative liabilities | 693 | 1,288
Fair Value, Measurements, Recurring | Interest rate contracts |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | 53 | 49
Derivative liabilities | 59 | 88
Fair Value, Measurements, Recurring | Cash and cash equivalents |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 47 | 25
Fair Value, Measurements, Recurring | U.S. government and federal agency obligations |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 43 | 73
Fair Value, Measurements, Recurring | Federal agency mortgage-backed securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 82 | 62
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 14 | 17
Fair Value, Measurements, Recurring | Corporate debt securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 99 | 84
Fair Value, Measurements, Recurring | Equity securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 334 | 292
Investments, Fair Value Disclosure | 68 | 54
Fair Value, Measurements, Recurring | Foreign government fixed income securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 5 | 3
Fair Value, Measurements, Recurring | Level 1 |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Debt securities | 0 | 0
Available-for-sale Securities | 3 | 10
U.S. government and federal agency obligations | 1 | 1
Total assets | 616 | 960
Total liabilities | 257 | 494
Fair Value, Measurements, Recurring | Level 1 | Commodity contracts |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | 191 | 560
Derivative liabilities | 257 | 494
Fair Value, Measurements, Recurring | Level 1 | Interest rate contracts |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | 0 | 0
Derivative liabilities | 0 | 0
Fair Value, Measurements, Recurring | Level 1 | Cash and cash equivalents |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 45 | 25
Fair Value, Measurements, Recurring | Level 1 | U.S. government and federal agency obligations |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 42 | 72
Fair Value, Measurements, Recurring | Level 1 | Federal agency mortgage-backed securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 1 | Commercial mortgage-backed securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 1 | Corporate debt securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 1 | Equity securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 334 | 292
Fair Value, Measurements, Recurring | Level 1 | Foreign government fixed income securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 2 |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Debt securities | 0 | 0
Available-for-sale Securities | 0 | 0
U.S. government and federal agency obligations | 0 | 0
Total assets | 765 | 765
Total liabilities | 418 | 724
Fair Value, Measurements, Recurring | Level 2 | Commodity contracts |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | 509 | 549
Derivative liabilities | 359 | 636
Fair Value, Measurements, Recurring | Level 2 | Interest rate contracts |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | 53 | 49
Derivative liabilities | 59 | 88
Fair Value, Measurements, Recurring | Level 2 | Cash and cash equivalents |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 2 | 0
Fair Value, Measurements, Recurring | Level 2 | U.S. government and federal agency obligations |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 1 | 1
Fair Value, Measurements, Recurring | Level 2 | Federal agency mortgage-backed securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 82 | 62
Fair Value, Measurements, Recurring | Level 2 | Commercial mortgage-backed securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 14 | 17
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 99 | 84
Fair Value, Measurements, Recurring | Level 2 | Equity securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 2 | Foreign government fixed income securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 5 | 3
Fair Value, Measurements, Recurring | Level 3 |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Debt securities | 19 | 17
Available-for-sale Securities | 0 | 0
U.S. government and federal agency obligations | 0 | 0
Total assets | 64 | 107
Total liabilities | 77 | 158
Fair Value, Measurements, Recurring | Level 3 | Commodity contracts |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | 45 | 90
Derivative liabilities | 77 | 158
Fair Value, Measurements, Recurring | Level 3 | Interest rate contracts |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Derivative assets | 0 | 0
Derivative liabilities | 0 | 0
Fair Value, Measurements, Recurring | Level 3 | Cash and cash equivalents |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 3 | U.S. government and federal agency obligations |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 3 | Federal agency mortgage-backed securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 3 | Commercial mortgage-backed securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 3 | Corporate debt securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 3 | Equity securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | 0 | 0
Fair Value, Measurements, Recurring | Level 3 | Foreign government fixed income securities |  |
Investment in available-for-sale securities (classified within other non-current assets): |  |
Nuclear trust fund investments | $ 0 | $ 0

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