v3.2.0.727
Segment Information (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues $ 1,442 $ 1,939 $ 3,088 $ 3,904
Commodity Margin 657 632 [1] 1,192 1,277 [1]
Add: Mark-to-market commodity activity, net and other 19 [1] 173 [2] 120 [3] 136 [3]
Plant operating expense 272 274 532 539
Depreciation and amortization expense 160 147 318 300
Sales, general and other administrative expense 30 38 67 71
Other operating expenses 20 21 40 43
(Income) loss from unconsolidated investments in power plants (7) (4) (12) (13)
Income from operations 201 329 367 473
Interest expense, net of interest income 157 167 310 332
Debt extinguishment costs and other (income) expense, net 18 6 39 17
Income before income taxes 26 156 18 124
Lease levelization (18) (27) (42) (56)
Contract amortization 3 3 7 7
West [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 421 488 938 981
Commodity Margin 240 228 [1] 458 430 [1]
Add: Mark-to-market commodity activity, net and other (14) [1] 21 [2] 105 [3] 50 [3]
Plant operating expense 120 95 226 200
Depreciation and amortization expense 65 58 132 118
Sales, general and other administrative expense 6 7 16 17
Other operating expenses 10 15 20 27
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations 25 74 169 118
Texas [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 575 963 1,159 1,622
Commodity Margin 170 177 [1] 319 298 [1]
Add: Mark-to-market commodity activity, net and other 10 [1] 184 [2] 51 [3] 138 [3]
Plant operating expense 82 83 171 173
Depreciation and amortization expense 50 48 99 90
Sales, general and other administrative expense 15 18 32 30
Other operating expenses 2 1 4 3
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations 31 211 64 140
East [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 453 519 1,005 1,363
Commodity Margin 247 227 [1] 415 549 [1]
Add: Mark-to-market commodity activity, net and other 30 [1] (24) [2] (22) [3] (35) [3]
Plant operating expense 77 103 149 182
Depreciation and amortization expense 45 40 87 91
Sales, general and other administrative expense 9 12 19 24
Other operating expenses 8 9 16 16
(Income) loss from unconsolidated investments in power plants (7) (4) (12) (13)
Income from operations 145 43 134 214
Geography Consolidation and Elimination [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues (7) (31) (14) (62)
Commodity Margin 0 0 [1] 0 0 [1]
Add: Mark-to-market commodity activity, net and other (7) [1] (8) [2] (14) [3] (17) [3]
Plant operating expense (7) (7) (14) (16)
Depreciation and amortization expense 0 1 0 1
Sales, general and other administrative expense 0 1 0 0
Other operating expenses 0 (4) 0 (3)
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations 0 1 0 1
Six Power Plants [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Commodity Margin [2]   42   81
Operating Segments [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 1,442 1,939 3,088 3,904
Operating Segments [Member] | West [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 421 487 936 978
Operating Segments [Member] | Texas [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 570 960 1,151 1,607
Operating Segments [Member] | East [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 451 492 1,001 1,319
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 0 1 2 3
Intersegment Eliminations [Member] | Texas [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 5 3 8 15
Intersegment Eliminations [Member] | East [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 2 27 4 44
Intersegment Eliminations [Member] | Geography Consolidation and Elimination [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues $ (7) $ (31) $ (14) $ (62)
Six Power Plants [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Number of power plants disposed       6
[1] Commodity Margin related to the six power plants sold in our East segment on July 3, 2014, was $42 million and $81 million for the three and six months ended June 30, 2014
[2] Includes $(18) million and $(27) million of lease levelization and $3 million and $3 million of amortization expense for the three months ended June 30, 2015 and 2014, respectively.
[3] Includes $(42) million and $(56) million of lease levelization and $7 million and $7 million of amortization expense for the six months ended June 30, 2015 and 2014, respectively.