v3.8.0.1
Segment Information (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues $ 2,586 $ 2,355 $ 6,951 $ 5,134
Commodity Margin 864 820 2,069 2,057
Add: Mark-to-market commodity activity, net and other 1 [1] 63 [1] (60) [2] (63) [2]
Plant operating expense 228 215 812 741
Depreciation and amortization expense 179 161 542 490
Sales, general and other administrative expense 37 33 117 106
Other operating expenses 23 18 63 55
Impairment losses 12 0 41 13
Gain on sale of assets, net 0 0 (27) 0
(Income) loss from unconsolidated investments in power plants (7) (6) (17) (16)
Income from operations 393 462 478 605
Interest expense, net of interest income 156 158 469 472
Debt Extinguishment Costs and Other (Income) Expense, Net 8 7 42 33
Income (loss) before income taxes 229 297 (33) 100
Lease levelization 33 40 (13) (2)
West [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 555 525 1,670 1,163
Commodity Margin 327 298 792 749
Add: Mark-to-market commodity activity, net and other (40) [1] 11 [1] (1) [2] (5) [2]
Plant operating expense 83 79 291 268
Depreciation and amortization expense 63 56 189 168
Sales, general and other administrative expense 10 9 31 27
Other operating expenses 13 8 30 23
Impairment losses 0   28 13
Gain on sale of assets, net     0  
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations 118 157 222 245
Texas [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 1,131 1,070 2,735 2,139
Commodity Margin 201 198 516 511
Add: Mark-to-market commodity activity, net and other 88 [1] 110 [1] 28 [2] 7 [2]
Plant operating expense 77 65 282 236
Depreciation and amortization expense 61 53 187 159
Sales, general and other administrative expense 16 13 54 43
Other operating expenses 6 2 12 6
Impairment losses 12   13 0
Gain on sale of assets, net     0  
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations 117 175 (4) 74
East [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 908 766 2,568 1,855
Commodity Margin 336 324 761 797
Add: Mark-to-market commodity activity, net and other (39) [1] (51) [1] (65) [2] (44) [2]
Plant operating expense 75 78 260 258
Depreciation and amortization expense 55 52 166 163
Sales, general and other administrative expense 10 12 31 36
Other operating expenses 6 7 23 27
Impairment losses 0   0 0
Gain on sale of assets, net     (27)  
(Income) loss from unconsolidated investments in power plants (7) (6) (17) (16)
Income from operations 158 130 260 285
Consolidation, Eliminations [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues (8) (6) (22) (23)
Commodity Margin 0 0 0 0
Add: Mark-to-market commodity activity, net and other (8) [1] (7) [1] (22) [2] (21) [2]
Plant operating expense (7) (7) (21) (21)
Depreciation and amortization expense 0 0 0 0
Sales, general and other administrative expense 1 (1) 1 0
Other operating expenses (2) 1 (2) (1)
Impairment losses 0   0 0
Gain on sale of assets, net     0  
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations 0 0 0 1
Operating Segments [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 2,586 2,355 6,951 5,134
Operating Segments [Member] | West [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 553 524 1,666 1,159
Operating Segments [Member] | Texas [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 1,127 1,067 2,723 2,129
Operating Segments [Member] | East [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 906 764 2,562 1,846
Operating Segments [Member] | Consolidation, Eliminations [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 2 1 4 4
Intersegment Eliminations [Member] | Texas [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 4 3 12 10
Intersegment Eliminations [Member] | East [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 2 2 6 9
Intersegment Eliminations [Member] | Consolidation, Eliminations [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues (8) (6) (22) (23)
Other Assets [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Amortization of Intangible Assets $ 39 $ 25 $ 143 $ 79
[1] Includes $33 million and $40 million of lease levelization and $39 million and $25 million of amortization expense for the three months ended September 30, 2017 and 2016, respectively.
[2] Includes $(13) million and $(2) million of lease levelization and $143 million and $79 million of amortization expense for the nine months ended September 30, 2017 and 2016, respectively.