v2.4.0.8
Segment Information (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues $ 2,187 $ 2,050 $ 6,091 $ 4,863
Commodity Margin 944 [1] 985 [1] 2,221 [1] 1,979 [1]
Add: Mark-to-market commodity activity, net and other (25) [2] 6 [2] 111 [2] 4 [2]
Plant operating expense 215 200 754 684
Depreciation and amortization expense 153 150 453 441
Sales, general and other administrative expense 37 33 108 102
Other operating expenses 23 20 66 58
Impairment losses 123 0 123 0
(Gain) on sale of assets, net (753) 0 (753) 0
(Income) loss from unconsolidated investments in power plants (5) (9) (18) (25)
Income from operations 1,126 597 1,599 723
Interest expense, net of interest income 154 174 486 517
Debt extinguishment costs and other (income) expense, net 344 7 361 83
Income before income taxes 628 416 752 123
Commodity Margin for Six Southeast Power Plants Sold   65 81 122
Lease levelization 49 44 (7) 17
Contract amortization 4 4 11 11
West [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 715 621 1,696 1,484
Commodity Margin 361 [1] 337 [1] 791 [1] 737 [1]
Add: Mark-to-market commodity activity, net and other 41 [2] 16 [2] 91 [2] (2) [2]
Plant operating expense 91 84 291 271
Depreciation and amortization expense 65 58 183 164
Sales, general and other administrative expense 11 9 28 24
Other operating expenses 12 12 39 33
Impairment losses 0   0  
(Gain) on sale of assets, net 0   0  
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations 223 190 341 243
Texas [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 989 836 2,611 1,796
Commodity Margin 346 [1] 328 [1] 644 [1] 537 [1]
Add: Mark-to-market commodity activity, net and other (64) [2] (5) [2] 74 [2] 18 [2]
Plant operating expense 77 56 250 214
Depreciation and amortization expense 51 41 141 125
Sales, general and other administrative expense 18 13 48 43
Other operating expenses 1 2 4 4
Impairment losses 0   0  
(Gain) on sale of assets, net 0   0  
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations 135 211 275 169
East [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 490 631 1,853 1,662
Commodity Margin 237 [1] 320 [1] 786 [1] 705 [1]
Add: Mark-to-market commodity activity, net and other 4 [2] 3 [2] (31) [2] 12 [2]
Plant operating expense 55 67 237 221
Depreciation and amortization expense 38 51 129 153
Sales, general and other administrative expense 8 10 32 34
Other operating expenses 6 9 22 25
Impairment losses 123   123  
(Gain) on sale of assets, net (753)   (753)  
(Income) loss from unconsolidated investments in power plants (5) (9) (18) (25)
Income from operations 769 195 983 309
Geography Consolidation and Elimination [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues (7) (38) (69) (79)
Commodity Margin 0 [1] 0 [1] 0 [1] 0 [1]
Add: Mark-to-market commodity activity, net and other (6) [2] (8) [2] (23) [2] (24) [2]
Plant operating expense (8) (7) (24) (22)
Depreciation and amortization expense (1) 0 0 (1)
Sales, general and other administrative expense 0 1 0 1
Other operating expenses 4 (3) 1 (4)
Impairment losses 0   0  
(Gain) on sale of assets, net 0   0  
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations (1) 1 0 2
Operating Segments [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 2,187 2,050 6,091 4,863
Operating Segments [Member] | West [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 714 620 1,692 1,482
Operating Segments [Member] | Texas [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 985 842 2,592 1,820
Operating Segments [Member] | East [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 488 588 1,807 1,561
Operating Segments [Member] | Geography Consolidation and Elimination [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 0 0 0 0
Intersegment Eliminations [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 0 0 0 0
Intersegment Eliminations [Member] | West [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 1 1 4 2
Intersegment Eliminations [Member] | Texas [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 4 (6) 19 (24)
Intersegment Eliminations [Member] | East [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 2 43 46 101
Intersegment Eliminations [Member] | Geography Consolidation and Elimination [Member]
        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues $ (7) $ (38) $ (69) $ (79)
[1] the three months ended September 30, 2014.
[2] Includes $49 million and $44 million of lease levelization and $4 million and $4 million of amortization expense for the three months ended September 30, 2014 and 2013, respectively.