v3.7.0.1
Segment Information (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues $ 2,281 $ 1,615
Commodity Margin 558 580
Add: Mark-to-market commodity activity, net and other [1] 31 (91)
Plant operating expense 282 255
Depreciation and amortization expense 206 180
Sales, general and other administrative expense 40 38
Other operating expenses 20 20
Gain on sale of assets, net (27) 0
(Income) loss from unconsolidated investments in power plants (4) (7)
Income from operations 72 3
Interest expense, net of interest income 159 157
Debt Extinguishment Costs and Other (Income) Expense, Net 26 5
Loss before income taxes (113) (159)
Lease levelization (22) (22)
West [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 714 426
Commodity Margin 221 197
Add: Mark-to-market commodity activity, net and other [1] 77 46
Plant operating expense 97 91
Depreciation and amortization expense 91 69
Sales, general and other administrative expense 13 10
Other operating expenses 9 8
Gain on sale of assets, net 0  
(Income) loss from unconsolidated investments in power plants 0 0
Income from operations 88 65
Texas [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 802 535
Commodity Margin 148 153
Add: Mark-to-market commodity activity, net and other [1] (30) (110)
Plant operating expense 96 86
Depreciation and amortization expense 62 53
Sales, general and other administrative expense 17 16
Other operating expenses 3 2
Gain on sale of assets, net 0  
(Income) loss from unconsolidated investments in power plants 0 0
Income from operations (60) (114)
East [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 772 662
Commodity Margin 189 230
Add: Mark-to-market commodity activity, net and other [1] (8) (21)
Plant operating expense 96 84
Depreciation and amortization expense 53 58
Sales, general and other administrative expense 10 12
Other operating expenses 9 10
Gain on sale of assets, net (27)  
(Income) loss from unconsolidated investments in power plants (4) (7)
Income from operations 44 52
Consolidation, Eliminations [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues (7) (8)
Commodity Margin 0 0
Add: Mark-to-market commodity activity, net and other [1] (8) (6)
Plant operating expense (7) (6)
Depreciation and amortization expense 0 0
Sales, general and other administrative expense 0 0
Other operating expenses (1) 0
Gain on sale of assets, net 0  
(Income) loss from unconsolidated investments in power plants 0 0
Income from operations 0 0
Operating Segments [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 2,281 1,615
Operating Segments [Member] | West [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 712 424
Operating Segments [Member] | Texas [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 799 532
Operating Segments [Member] | East [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 770 659
Operating Segments [Member] | Consolidation, Eliminations [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 0 0
Intersegment Eliminations [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 0 0
Intersegment Eliminations [Member] | West [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 2 2
Intersegment Eliminations [Member] | Texas [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 3 3
Intersegment Eliminations [Member] | East [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues 2 3
Intersegment Eliminations [Member] | Consolidation, Eliminations [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Operating revenues (7) (8)
Other Assets [Member]    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Amortization of Intangible Assets $ 60 $ 27
[1] Includes $(22) million and $(22) million of lease levelization and $60 million and $27 million of amortization expense for the three months ended March 31, 2017 and 2016, respectively.