v3.7.0.1
Segment Information (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues $ 2,084 $ 1,164 $ 4,365 $ 2,779
Commodity Margin 647 657 1,205 1,237
Add: Mark-to-market commodity activity, net and other (92) [1] (35) [1] (61) [2] (126) [2]
Plant operating expense 302 271 584 526
Depreciation and amortization expense 186 162 392 342
Sales, general and other administrative expense 40 35 80 73
Other operating expenses 20 17 40 37
Gain on sale of assets, net 0 0 (27) 0
(Income) loss from unconsolidated investments in power plants (6) (3) (10) (10)
Income from operations 13 140 85 143
Interest expense, net of interest income 154 157 313 314
Debt Extinguishment Costs and Other (Income) Expense, Net 8 21 34 26
Loss before income taxes (149) (38) (262) (197)
Lease levelization (24) (20) (46) (42)
West [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 401 212 1,115 638
Commodity Margin 244 254 465 451
Add: Mark-to-market commodity activity, net and other (38) [1] (62) [1] 39 [2] (16) [2]
Plant operating expense 111 98 208 189
Depreciation and amortization expense 63 56 154 125
Sales, general and other administrative expense 8 8 21 18
Other operating expenses 8 7 17 15
Gain on sale of assets, net     0  
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations 16 23 104 88
Texas [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 802 534 1,604 1,069
Commodity Margin 167 160 315 313
Add: Mark-to-market commodity activity, net and other (30) [1] 7 [1] (60) [2] (103) [2]
Plant operating expense 109 85 205 171
Depreciation and amortization expense 65 53 127 106
Sales, general and other administrative expense 21 14 38 30
Other operating expenses 3 2 6 4
Gain on sale of assets, net     0  
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations (61) 13 (121) (101)
East [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 888 427 1,660 1,089
Commodity Margin 236 243 425 473
Add: Mark-to-market commodity activity, net and other (18) [1] 28 [1] (26) [2] 7 [2]
Plant operating expense 89 96 185 180
Depreciation and amortization expense 58 53 111 111
Sales, general and other administrative expense 11 12 21 24
Other operating expenses 8 10 17 20
Gain on sale of assets, net     (27)  
(Income) loss from unconsolidated investments in power plants (6) (3) (10) (10)
Income from operations 58 103 102 155
Consolidation, Eliminations [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues (7) (9) (14) (17)
Commodity Margin 0 0 0 0
Add: Mark-to-market commodity activity, net and other (6) [1] (8) [1] (14) [2] (14) [2]
Plant operating expense (7) (8) (14) (14)
Depreciation and amortization expense 0 0 0 0
Sales, general and other administrative expense 0 1 0 1
Other operating expenses 1 (2) 0 (2)
Gain on sale of assets, net     0  
(Income) loss from unconsolidated investments in power plants 0 0 0 0
Income from operations 0 1 0 1
Operating Segments [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 2,084 1,164 4,365 2,779
Operating Segments [Member] | West [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 401 211 1,113 635
Operating Segments [Member] | Texas [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 797 530 1,596 1,062
Operating Segments [Member] | East [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 886 423 1,656 1,082
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 0 1 2 3
Intersegment Eliminations [Member] | Texas [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 5 4 8 7
Intersegment Eliminations [Member] | East [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues 2 4 4 7
Intersegment Eliminations [Member] | Consolidation, Eliminations [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Operating revenues (7) (9) (14) (17)
Other Assets [Member]        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Amortization of Intangible Assets $ 44 $ 27 $ 104 $ 54
[1] Includes $(24) million and $(20) million of lease levelization and $44 million and $27 million of amortization expense for the three months ended June 30, 2017 and 2016, respectively.
[2] Includes $(46) million and $(42) million of lease levelization and $104 million and $54 million of amortization expense for the six months ended June 30, 2017 and 2016, respectively.