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Fair Value of Financial Instruments - Assets and Liabilities Measured and Recorded at Fair Value (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets $ 1,277 $ 798
Derivative liabilities 994 752
Commodity contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 1,169 745
Derivative liabilities 971 693
Interest rate contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 108 53
Derivative liabilities 23 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 1,994 1,513
Derivative liabilities 994 752
Fair value, measurements, recurring | Commodity contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 1,169 745
Derivative liabilities 971 693
Fair value, measurements, recurring | Interest rate contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 108 53
Derivative liabilities 23 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 25 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 42 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 97 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 101 99
Fair value, measurements, recurring | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Nuclear trust fund investments 342 334
Equity contracts 65 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 6 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 601 616
Derivative liabilities 236 257
Fair value, measurements, recurring | Level 1 | Commodity contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 188 191
Derivative liabilities 236 257
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 25 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 42 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 342 334
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 809 765
Derivative liabilities 411 418
Fair value, measurements, recurring | Level 2 | Commodity contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 481 509
Derivative liabilities 388 359
Fair value, measurements, recurring | Level 2 | Interest rate contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 108 53
Derivative liabilities 23 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 0 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 97 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 101 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 6 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 519 64
Derivative liabilities 347 77
Fair value, measurements, recurring | Level 3 | Commodity contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 500 45
Derivative liabilities 347 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