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Fair Value of Financial Instruments - Assets and liabilities measured and recorded at fair value (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets $ 1,369 $ 798
Derivative liabilities 1,054 752
Commodity contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 1,277 745
Derivative liabilities 1,024 693
Interest rate contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 92 53
Derivative liabilities 30 59
Fair value, measurements, recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments in securities (classified within other non-current assets) 22 22
U.S. government and federal agency obligations 1 1
Total assets 2,072 1,513
Derivative liabilities 1,054 752
Fair value, measurements, recurring | Commodity contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 1,277 745
Derivative liabilities 1,024 693
Fair value, measurements, recurring | Interest rate contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 92 53
Derivative liabilities 30 59
Fair value, measurements, recurring | Cash and cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 16 47
Fair value, measurements, recurring | U.S. government and federal agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 56 43
Fair value, measurements, recurring | Federal agency mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 92 82
Fair value, measurements, recurring | Commercial mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 16 14
Fair value, measurements, recurring | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 100 99
Fair value, measurements, recurring | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 333 334
Equity contracts 62 68
Fair value, measurements, recurring | Foreign government fixed income securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 5 5
Fair value, measurements, recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments in securities (classified within other non-current assets) 3 3
U.S. government and federal agency obligations 1 1
Total assets 576 616
Derivative liabilities 224 257
Fair value, measurements, recurring | Level 1 | Commodity contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 168 191
Derivative liabilities 224 257
Fair value, measurements, recurring | Level 1 | Interest rate contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 0 0
Derivative liabilities 0 0
Fair value, measurements, recurring | Level 1 | Cash and cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 16 45
Fair value, measurements, recurring | Level 1 | U.S. government and federal agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 55 42
Fair value, measurements, recurring | Level 1 | Federal agency mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 1 | Commercial mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 1 | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 1 | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 333 334
Fair value, measurements, recurring | Level 1 | Foreign government fixed income securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments in securities (classified within other non-current assets) 0 0
U.S. government and federal agency obligations 0 0
Total assets 1,366 765
Derivative liabilities 759 418
Fair value, measurements, recurring | Level 2 | Commodity contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 1,060 509
Derivative liabilities 729 359
Fair value, measurements, recurring | Level 2 | Interest rate contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 92 53
Derivative liabilities 30 59
Fair value, measurements, recurring | Level 2 | Cash and cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 2
Fair value, measurements, recurring | Level 2 | U.S. government and federal agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 1 1
Fair value, measurements, recurring | Level 2 | Federal agency mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 92 82
Fair value, measurements, recurring | Level 2 | Commercial mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 16 14
Fair value, measurements, recurring | Level 2 | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 100 99
Fair value, measurements, recurring | Level 2 | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 2 | Foreign government fixed income securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 5 5
Fair value, measurements, recurring | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments in securities (classified within other non-current assets) 19 19
U.S. government and federal agency obligations 0 0
Total assets 68 64
Derivative liabilities 71 77
Fair value, measurements, recurring | Level 3 | Commodity contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 49 45
Derivative liabilities 71 77
Fair value, measurements, recurring | Level 3 | Interest rate contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 0 0
Derivative liabilities 0 0
Fair value, measurements, recurring | Level 3 | Cash and cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 3 | U.S. government and federal agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 3 | Federal agency mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 3 | Commercial mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 3 | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 3 | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 0 0
Fair value, measurements, recurring | Level 3 | Foreign government fixed income securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments $ 0 $ 0