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Segment Information - Generation Total Revenues (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Segment Reporting Information [Line Items]        
Revenues $ 7,689 $ 8,076 $ 17,166 $ 17,769
Operating revenues from affiliates 0 0 0 0
Generation Mid Atlantic [Member]        
Segment Reporting Information [Line Items]        
Revenues 1,189 1,282 2,445 2,722
Generation Midwest [Member]        
Segment Reporting Information [Line Items]        
Revenues 1,034 1,130 2,142 2,475
Generation New York [Member]        
Segment Reporting Information [Line Items]        
Revenues 390 392 782 801
Generation ERCOT [Member]        
Segment Reporting Information [Line Items]        
Revenues 229 277 441 486
Generation Other Regions [Member]        
Segment Reporting Information [Line Items]        
Revenues 861 821 2,214 1,902
Generation Reportable Segments Total [Member]        
Segment Reporting Information [Line Items]        
Revenues 3,703 3,902 8,024 8,386
Generation Natural Gas [Member]        
Segment Reporting Information [Line Items]        
Revenues 348 526 1,242 1,506
Generation All Other Segments [Member]        
Segment Reporting Information [Line Items]        
Revenues 159 151 240 198
Unrealized Gain (Loss) on Securities 38 5 (102) 175
Generation Total Consolidated Group [Member]        
Segment Reporting Information [Line Items]        
Revenues 4,210 4,579 9,506 10,090
Operating Segments [Member]        
Segment Reporting Information [Line Items]        
Revenues 7,689 8,076 17,166 17,769
Operating Segments [Member] | Generation Mid Atlantic [Member]        
Segment Reporting Information [Line Items]        
Revenues 1,183 1,278 2,446 2,712
Operating Segments [Member] | Generation Midwest [Member]        
Segment Reporting Information [Line Items]        
Revenues 1,042 1,135 2,156 2,479
Operating Segments [Member] | Generation New York [Member]        
Segment Reporting Information [Line Items]        
Revenues 390 390 782 800
Operating Segments [Member] | Generation ERCOT [Member]        
Segment Reporting Information [Line Items]        
Revenues 225 276 433 484
Operating Segments [Member] | Generation Other Regions [Member]        
Segment Reporting Information [Line Items]        
Revenues 878 860 2,235 1,973
Operating Segments [Member] | Generation Reportable Segments Total [Member]        
Segment Reporting Information [Line Items]        
Revenues 3,718 3,939 8,052 8,448
Operating Segments [Member] | Generation Natural Gas [Member]        
Segment Reporting Information [Line Items]        
Revenues 333 489 1,214 1,444
Operating Segments [Member] | Generation All Other Segments [Member]        
Segment Reporting Information [Line Items]        
Unrealized Gain (Loss) on Securities (74) 90 (14) 102
Operating Segments [Member] | Generation Total Consolidated Group [Member]        
Segment Reporting Information [Line Items]        
Revenues 4,210 4,579 9,506 10,090
Corporate, Non-Segment [Member] | Generation All Other Segments [Member]        
Segment Reporting Information [Line Items]        
Revenues 159 151 240 198
Intersegment Eliminations [Member]        
Segment Reporting Information [Line Items]        
Revenues (751) (779) (1,537) (1,655)
Operating revenues from affiliates (750) (776) (1,534) (1,650)
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]        
Segment Reporting Information [Line Items]        
Operating revenues from affiliates 6 4 (1) 10
Intersegment Eliminations [Member] | Generation Midwest [Member]        
Segment Reporting Information [Line Items]        
Operating revenues from affiliates (8) (5) (14) (4)
Intersegment Eliminations [Member] | Generation New York [Member]        
Segment Reporting Information [Line Items]        
Operating revenues from affiliates 0 2 0 1
Intersegment Eliminations [Member] | Generation ERCOT [Member]        
Segment Reporting Information [Line Items]        
Operating revenues from affiliates 4 1 8 2
Intersegment Eliminations [Member] | Generation Other Regions [Member]        
Segment Reporting Information [Line Items]        
Operating revenues from affiliates (17) (39) (21) (71)
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]        
Segment Reporting Information [Line Items]        
Operating revenues from affiliates (15) (37) (28) (62)
Intersegment Eliminations [Member] | Generation Natural Gas [Member]        
Segment Reporting Information [Line Items]        
Operating revenues from affiliates 15 37 28 62
Intersegment Eliminations [Member] | Generation All Other Segments [Member]        
Segment Reporting Information [Line Items]        
Operating revenues from affiliates 0 0 0 0
Intersegment Eliminations [Member] | Generation Total Consolidated Group [Member]        
Segment Reporting Information [Line Items]        
Operating revenues from affiliates 0 0 0 0
Revenue from Contract with Customer [Member] | Operating Segments [Member] | Generation Mid Atlantic [Member]        
Segment Reporting Information [Line Items]        
Revenues 1,162 1,220 2,448 2,574
Revenue from Contract with Customer [Member] | Operating Segments [Member] | Generation Midwest [Member]        
Segment Reporting Information [Line Items]        
Revenues 974 1,062 2,030 2,336
Revenue from Contract with Customer [Member] | Operating Segments [Member] | Generation New York [Member]        
Segment Reporting Information [Line Items]        
Revenues 373 392 781 831
Revenue from Contract with Customer [Member] | Operating Segments [Member] | Generation ERCOT [Member]        
Segment Reporting Information [Line Items]        
Revenues 178 165 307 315
Revenue from Contract with Customer [Member] | Operating Segments [Member] | Generation Other Regions [Member]        
Segment Reporting Information [Line Items]        
Revenues 814 761 1,976 1,696
Revenue from Contract with Customer [Member] | Operating Segments [Member] | Generation Reportable Segments Total [Member]        
Segment Reporting Information [Line Items]        
Revenues 3,501 3,600 7,542 7,752
Revenue from Contract with Customer [Member] | Operating Segments [Member] | Generation Natural Gas [Member]        
Segment Reporting Information [Line Items]        
Revenues 177 295 763 816
Revenue from Contract with Customer [Member] | Operating Segments [Member] | Generation Total Consolidated Group [Member]        
Segment Reporting Information [Line Items]        
Revenues 3,786 4,020 8,535 8,826
Revenue from Contract with Customer [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]        
Segment Reporting Information [Line Items]        
Revenues 108 125 230 258
Other [Member] | Operating Segments [Member] | Generation Mid Atlantic [Member]        
Segment Reporting Information [Line Items]        
Revenues 21 58 (2) 138
Other [Member] | Operating Segments [Member] | Generation Midwest [Member]        
Segment Reporting Information [Line Items]        
Revenues 68 73 126 143
Other [Member] | Operating Segments [Member] | Generation New York [Member]        
Segment Reporting Information [Line Items]        
Revenues 17 (2) 1 (31)
Other [Member] | Operating Segments [Member] | Generation ERCOT [Member]        
Segment Reporting Information [Line Items]        
Revenues 47 111 126 169
Other [Member] | Operating Segments [Member] | Generation Other Regions [Member]        
Segment Reporting Information [Line Items]        
Revenues 64 99 259 277
Other [Member] | Operating Segments [Member] | Generation Reportable Segments Total [Member]        
Segment Reporting Information [Line Items]        
Revenues 217 339 510 696
Other [Member] | Operating Segments [Member] | Generation Natural Gas [Member]        
Segment Reporting Information [Line Items]        
Revenues 156 194 451 628
Other [Member] | Operating Segments [Member] | Generation Total Consolidated Group [Member]        
Segment Reporting Information [Line Items]        
Revenues 424 559 971 1,264
Other [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]        
Segment Reporting Information [Line Items]        
Revenues $ 51 $ 26 $ 10 $ (60)