Segment Information - Generation Total Revenues (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Revenues		$ 8,757	$ 7,573
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues		1,425	1,520
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues		1,053	1,095
Generation New England [Member]
Segment Reporting Information [Line Items]
Revenues		547	470
Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues		307	203
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues		191	163
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues		184	223
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues		3,707	3,674
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues		1,181	1,065
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues		4,888	4,739
Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	[1],[2]	1,429	1,532
Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	[2]	1,051	1,089
Operating Segments [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Revenues	[2]	549	471
Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	[1],[2]	310	218
Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	[2]	192	163
Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	[2],[3]	189	222
Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	[2]	3,720	3,695
Operating Segments [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Amortization of intangible assets related to commodity contracts		(3)	20
Unrealized Gain (Loss) on Securities		44	63
Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	[2]	4,888	4,739
Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	[2],[4]	1,168	1,044
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues from affiliates		0	(1)
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Operating revenues from affiliates	[1]	(4)	(12)
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Operating revenues from affiliates		2	6
Intersegment Eliminations [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Operating revenues from affiliates		(2)	(1)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Operating revenues from affiliates	[1]	(3)	(15)
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Operating revenues from affiliates		(1)	0
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Operating revenues from affiliates	[3]	(5)	1
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Operating revenues from affiliates		(13)	(21)
Intersegment Eliminations [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues from affiliates	[4]	13	21
Intersegment Eliminations [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Operating revenues from affiliates		$ 0	$ 0

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[2] {F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOjk4YjY2MmVjNTk5ZDQ4ZGNhYWRkOTNkYjIyYTc3MzFkfFRleHRTZWxlY3Rpb246RDM5OTkzNTIwRTlBNTQ4OEJFQzlGRTdDMDZERTMzQUIM}

[3] {F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOjk4YjY2MmVjNTk5ZDQ4ZGNhYWRkOTNkYjIyYTc3MzFkfFRleHRTZWxlY3Rpb246QTZBNDlCQjFFQkUwNUI0QjgwNTJERUMxRjVGRjhFQjgM}

[4] {F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOjk4YjY2MmVjNTk5ZDQ4ZGNhYWRkOTNkYjIyYTc3MzFkfFRleHRTZWxlY3Rpb246OUZFMzMxN0MyRkRCNUZCRjg4REJDODEyRjhCMjQyRDIM}