v3.3.1.900
Segment and Significant Customer Information (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Sep. 30, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues $ 1,436 $ 1,948 $ 1,442 $ 1,646 $ 1,939 $ 2,187 $ 1,939 $ 1,965   $ 6,472 $ 8,030 $ 6,301
Commodity Margin                   2,786 2,759 [1] 2,568 [1]
Mark-to-Market Commodity Activity, Net and Other Revenue [2]                   (81) 379 (3)
Plant operating expense                   1,018 969 895
Depreciation and amortization expense                   638 603 593
Sales, general and other administrative expense                   138 144 136
Other Cost and Expense, Operating                   80 88 81
Impairment losses                   0 123 16
Gain (Loss) on Disposition of Assets                   0 753 0
(Income) from unconsolidated investments in power plants                   (24) (25) (30)
Income from operations $ 22 $ 466 $ 201 $ 166 $ 390 $ 1,126 $ 329 $ 144   855 1,989 874
Interest expense, net of interest income                   624 639 690
Debt Extinguishment Costs and Other (Income) Expense, Net                   58 367 164
Income before income taxes                   173 983 20
Lease levelization                   (2) (5) 6
Contract amortization                   $ 20 $ 14 $ 14
Number of significant customers                   two one two
West [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   $ 2,094 $ 2,358 $ 1,942
Commodity Margin                   1,106 1,050 [1] 1,020 [1]
Mark-to-Market Commodity Activity, Net and Other Revenue [2]                   160 220 (50)
Plant operating expense                   416 385 365
Depreciation and amortization expense                   250 245 227
Sales, general and other administrative expense                   35 41 37
Other Cost and Expense, Operating                   37 50 45
Impairment losses                     0 16
Gain (Loss) on Disposition of Assets                     0  
(Income) from unconsolidated investments in power plants                   0 0 0
Income from operations                   528 549 280
Texas [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   2,359 3,252 2,343
Commodity Margin                   736 760 [1] 632 [1]
Mark-to-Market Commodity Activity, Net and Other Revenue [2]                   (120) 142 51
Plant operating expense                   338 313 269
Depreciation and amortization expense                   204 191 165
Sales, general and other administrative expense                   63 64 56
Other Cost and Expense, Operating                   9 5 3
Impairment losses                     0 0
Gain (Loss) on Disposition of Assets                     0  
(Income) from unconsolidated investments in power plants                   0 0 0
Income from operations                   2 329 190
East [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   2,047 2,496 2,134
Commodity Margin                   944 949 [1] 916 [1]
Mark-to-Market Commodity Activity, Net and Other Revenue [2]                   (92) 48 27
Plant operating expense                   292 302 292
Depreciation and amortization expense                   184 168 203
Sales, general and other administrative expense                   40 39 42
Other Cost and Expense, Operating                   36 32 33
Impairment losses                     123 0
Gain (Loss) on Disposition of Assets                     753  
(Income) from unconsolidated investments in power plants                   (24) (25) (30)
Income from operations                   324 1,111 403
Commodity Margin for Six Southeast Power Plants Sold                     81 152
Number of power plants disposed of                 6      
Geography Consolidation and Elimination [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   (28) (76) (118)
Commodity Margin                   0 0 0
Mark-to-Market Commodity Activity, Net and Other Revenue                   (29) (31) (31)
Plant operating expense                   (28) (31) (31)
Depreciation and amortization expense                   0 (1) (2)
Sales, general and other administrative expense                   0 0 1
Other Cost and Expense, Operating                   (2) 1 0
Impairment losses                     0 0
Gain (Loss) on Disposition of Assets                     0  
(Income) from unconsolidated investments in power plants                   0 0 0
Income from operations                   1 0 1
PJM Settlement, Inc. [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   724 1,000 820
Pacific Gas & Electric Company [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   642   694
Operating Segments [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   6,472 8,030 6,301
Operating Segments [Member] | West [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   2,089 2,352 1,937
Operating Segments [Member] | Texas [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   2,344 3,229 2,347
Operating Segments [Member] | East [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   2,039 2,449 2,017
Operating Segments [Member] | Geography Consolidation and Elimination [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   0 0 0
Intersegment Eliminations [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   0 0 0
Intersegment Eliminations [Member] | West [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   5 6 5
Intersegment Eliminations [Member] | Texas [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   15 23 (4)
Intersegment Eliminations [Member] | East [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   8 47 117
Intersegment Eliminations [Member] | Geography Consolidation and Elimination [Member]                        
Revenues from External Customers and Long-Lived Assets [Line Items]                        
Operating revenues                   $ (28) $ (76) $ (118)
[1] Our East segment includes Commodity Margin of $81 million and $152 million for the years ended December 31, 2014 and 2013, respectively, related to the six power plants in our East segment that were sold in July 2014.
[2] Includes $(2) million, $(5) million and $6 million of lease levelization and $20 million, $14 million and $14 million of amortization expense for the years ended December 31, 2015, 2014 and 2013, respectively.