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Fair Value (Details) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Assets [Abstract]    
Nuclear decommissioning trusts $ (1,139) $ (1,037)
Liabilities [Abstract]    
Derivative Liabilities, Current 58 125
Derivative Liabilities, Noncurrent 21 26
Fair Value, Measurements, Recurring [Member]
    
Assets [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure (68) (123)
Nuclear decommissioning trusts (1,139) (1,037)
Other investments (124) (110)
Derivative assets [Abstract]    
Derivative Assets (98) (147)
Assets, Fair Value Disclosure (1,429) (1,417)
Liabilities [Abstract]    
Interest Rate Derivative Liabilities, at Fair Value 0 1
Derivative Liabilities 79 151
Financial and Nonfinancial Liabilities, Fair Value Disclosure 1,350 1,266
Fair Value, Measurements, Recurring [Member] | Natural Gas Commodity Contract [Member]
    
Derivative assets [Abstract]    
Derivative Assets (9) (40)
Liabilities [Abstract]    
Derivative Liabilities 26 56
Fair Value, Measurements, Recurring [Member] | Electricity [Member]
    
Derivative assets [Abstract]    
Derivative Assets (82) (102)
Liabilities [Abstract]    
Derivative Liabilities 51 93
Fair Value, Measurements, Recurring [Member] | Other Commodity Contract [Member]
    
Derivative assets [Abstract]    
Derivative Assets (7) (5)
Liabilities [Abstract]    
Derivative Liabilities 2 1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
    
Assets [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure 0 0
Nuclear decommissioning trusts (757) (694)
Other investments (76) (66)
Derivative assets [Abstract]    
Derivative Assets (436) (561)
Assets, Fair Value Disclosure (1,269) (1,321)
Liabilities [Abstract]    
Interest Rate Derivative Liabilities, at Fair Value 0 0
Derivative Liabilities 426 532
Financial and Nonfinancial Liabilities, Fair Value Disclosure 843 789
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Natural Gas Commodity Contract [Member]
    
Derivative assets [Abstract]    
Derivative Assets (388) (555)
Liabilities [Abstract]    
Derivative Liabilities 374 526
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Electricity [Member]
    
Derivative assets [Abstract]    
Derivative Assets 0 0
Liabilities [Abstract]    
Derivative Liabilities 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Commodity Contract [Member]
    
Derivative assets [Abstract]    
Derivative Assets (48) (6)
Liabilities [Abstract]    
Derivative Liabilities 52 6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
    
Assets [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure (68) (123)
Nuclear decommissioning trusts (382) (343)
Other investments (48) (44)
Derivative assets [Abstract]    
Derivative Assets (280) (295)
Assets, Fair Value Disclosure (778) (805)
Liabilities [Abstract]    
Interest Rate Derivative Liabilities, at Fair Value 0 1
Derivative Liabilities 258 315
Financial and Nonfinancial Liabilities, Fair Value Disclosure 520 490
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Natural Gas Commodity Contract [Member]
    
Derivative assets [Abstract]    
Derivative Assets (61) (66)
Liabilities [Abstract]    
Derivative Liabilities 49 73
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Electricity [Member]
    
Derivative assets [Abstract]    
Derivative Assets (210) (226)
Liabilities [Abstract]    
Derivative Liabilities 204 240
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Commodity Contract [Member]
    
Derivative assets [Abstract]    
Derivative Assets (9) (3)
Liabilities [Abstract]    
Derivative Liabilities 5 1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
    
Assets [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure 0 0
Nuclear decommissioning trusts 0 0
Other investments 0 0
Derivative assets [Abstract]    
Derivative Assets (127) (160)
Assets, Fair Value Disclosure (127) (160)
Liabilities [Abstract]    
Interest Rate Derivative Liabilities, at Fair Value 0 0
Derivative Liabilities 131 173
Financial and Nonfinancial Liabilities, Fair Value Disclosure (4) (13)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Natural Gas Commodity Contract [Member]
    
Derivative assets [Abstract]    
Derivative Assets (34) (24)
Liabilities [Abstract]    
Derivative Liabilities 48 62
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Electricity [Member]
    
Derivative assets [Abstract]    
Derivative Assets (88) (134)
Liabilities [Abstract]    
Derivative Liabilities 83 111
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Commodity Contract [Member]
    
Derivative assets [Abstract]    
Derivative Assets (5) (2)
Liabilities [Abstract]    
Derivative Liabilities 0 0
Fair Value, Measurements, Recurring [Member] | Netting [Member]
    
Assets [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure 0 0
Nuclear decommissioning trusts 0 0
Other investments 0 0
Derivative assets [Abstract]    
Derivative Assets (745) (869)
Assets, Fair Value Disclosure (745) (869)
Liabilities [Abstract]    
Interest Rate Derivative Liabilities, at Fair Value 0 0
Derivative Liabilities 736 869
Financial and Nonfinancial Liabilities, Fair Value Disclosure 9 0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Natural Gas Commodity Contract [Member]
    
Derivative assets [Abstract]    
Derivative Assets (474) (605)
Liabilities [Abstract]    
Derivative Liabilities 445 605
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Electricity [Member]
    
Derivative assets [Abstract]    
Derivative Assets (216) (258)
Liabilities [Abstract]    
Derivative Liabilities 236 258
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Other Commodity Contract [Member]
    
Derivative assets [Abstract]    
Derivative Assets (55) (6)
Liabilities [Abstract]    
Derivative Liabilities 55 6
Current Asset [Member] | Fair Value, Measurements, Recurring [Member]
    
Derivative assets [Abstract]    
Assets, Fair Value Disclosure (140) (231)
Current Asset [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
    
Derivative assets [Abstract]    
Assets, Fair Value Disclosure (400) (493)
Current Asset [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
    
Derivative assets [Abstract]    
Assets, Fair Value Disclosure (285) (372)
Current Asset [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
    
Derivative assets [Abstract]    
Assets, Fair Value Disclosure (94) (120)
Current Asset [Member] | Fair Value, Measurements, Recurring [Member] | Netting [Member]
    
Derivative assets [Abstract]    
Assets, Fair Value Disclosure (639) (754)
Non Current Asset [Member] | Fair Value, Measurements, Recurring [Member]
    
Derivative assets [Abstract]    
Assets, Fair Value Disclosure (1,289) (1,186)
Non Current Asset [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
    
Derivative assets [Abstract]    
Assets, Fair Value Disclosure (869) (828)
Non Current Asset [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
    
Derivative assets [Abstract]    
Assets, Fair Value Disclosure (493) (433)
Non Current Asset [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
    
Derivative assets [Abstract]    
Assets, Fair Value Disclosure (33) (40)
Non Current Asset [Member] | Fair Value, Measurements, Recurring [Member] | Netting [Member]
    
Derivative assets [Abstract]    
Assets, Fair Value Disclosure (106) (115)
Current Liability [Member] | Fair Value, Measurements, Recurring [Member]
    
Liabilities [Abstract]    
Derivative Liabilities, Current 58 125
Current Liability [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
    
Liabilities [Abstract]    
Derivative Liabilities, Current 379 466
Current Liability [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
    
Liabilities [Abstract]    
Derivative Liabilities, Current 189 269
Current Liability [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
    
Liabilities [Abstract]    
Derivative Liabilities, Current 100 144
Current Liability [Member] | Fair Value, Measurements, Recurring [Member] | Netting [Member]
    
Liabilities [Abstract]    
Derivative Liabilities, Current 610 754
Noncurrent Liability [Member] | Fair Value, Measurements, Recurring [Member]
    
Liabilities [Abstract]    
Derivative Liabilities, Noncurrent 21 26
Noncurrent Liability [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
    
Liabilities [Abstract]    
Derivative Liabilities, Noncurrent 47 66
Noncurrent Liability [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
    
Liabilities [Abstract]    
Derivative Liabilities, Noncurrent 69 46
Noncurrent Liability [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
    
Liabilities [Abstract]    
Derivative Liabilities, Noncurrent 31 29
Noncurrent Liability [Member] | Fair Value, Measurements, Recurring [Member] | Netting [Member]
    
Liabilities [Abstract]    
Derivative Liabilities, Noncurrent 126 115
Other Investments [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
    
Assets [Abstract]    
Cash and Cash Equivalents, Fair Value Disclosure (17) (14)
Other Investments [Member] | Equity [Member] | Fair Value, Measurements, Recurring [Member]
    
Assets [Abstract]    
Other investments $ (124) $ (110)