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Summary of Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 2,075		$ 5,766
Mark-to-market for economic hedging activities	(69)	$ (28)	62	$ 9
Contract amortization	5		12
Operating revenues	3,061	2,740	7,795	7,246
Lease revenue	6		16
Derivative revenue	(65)	(33)	79	2
Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(480)		(918)
Contract amortization	0		0
Operating revenues	(722)	(509)	(928)	(924)
Lease revenue	0		0
Energy revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	479		1,403
Derivative revenue	871		1,884
Energy revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(479)		(890)
Derivative revenue	0		0
Capacity revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	254		688
Derivative revenue	45		110
Capacity revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1		0
Derivative revenue	0		0
Retail revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers
Retail revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers
Retail revenue | Mass customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,766		4,317
Retail revenue | Mass customers | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(2)		(4)
Retail revenue | Business Solutions customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	434		1,181
Retail revenue | Business Solutions customers | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Total retail revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,200		5,498
Total retail revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(2)		(4)
Derivative revenue
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	55		(31)
Derivative revenue	975		2,001
Derivative revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	(241)		(21)
Derivative revenue	(241)		0
Other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	68		225
Other revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		(3)
Total operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues	3,061		7,795
Total operating revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(721)		(918)
Other revenue, derivative
Disaggregation of Revenue [Line Items]
Derivative revenue	4		17
Other revenue, derivative | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Derivative revenue	0		0
Retail | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,199		5,492
Contract amortization	0		0
Operating revenues	2,203	1,936	5,497	4,868
Lease revenue	3		10
Retail | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(1)	(3)	(7)	(3)
Retail | Energy revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Derivative revenue	0		0
Retail | Capacity revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Derivative revenue	0		0
Retail | Retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers
Retail | Retail revenue | Mass customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,768		4,321
Retail | Retail revenue | Business Solutions customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	434		1,181
Retail | Total retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,202		5,502
Retail | Derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	1		(5)
Derivative revenue	1		(5)
Retail | Other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Retail | Total operating revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues	2,203		5,497
Retail | Other revenue, derivative | Operating Segments
Disaggregation of Revenue [Line Items]
Lease revenue	3
Derivative revenue	0		0
Generation | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	356		1,192
Contract amortization	5		12
Operating revenues	1,579	1,313	3,216	3,289
Lease revenue	3		6
Generation | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(740)	$ (493)	(944)	$ (872)
Generation | Energy revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	958		2,293
Derivative revenue	871		1,884
Generation | Capacity revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	253		688
Derivative revenue	45		110
Generation | Retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers
Generation | Retail revenue | Mass customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Generation | Retail revenue | Business Solutions customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Generation | Total retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Generation | Derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	295		(5)
Derivative revenue	1,215		2,006
Generation | Other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	68		228
Generation | Total operating revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues	1,579		3,216
Generation | Other revenue, derivative | Operating Segments
Disaggregation of Revenue [Line Items]
Derivative revenue	4		17
Generation | Gulf Coast | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(110)		45
Contract amortization	5		12
Lease revenue	0		0
Generation | Gulf Coast | Energy revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	680		1,558
Derivative revenue	896		1,877
Generation | Gulf Coast | Capacity revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	66		201
Derivative revenue	0		0
Generation | Gulf Coast | Retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers
Generation | Gulf Coast | Retail revenue | Mass customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Generation | Gulf Coast | Retail revenue | Business Solutions customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Generation | Gulf Coast | Total retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Generation | Gulf Coast | Derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	268		(7)
Derivative revenue	1,160		1,871
Generation | Gulf Coast | Other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	36		164
Generation | Gulf Coast | Total operating revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues	1,055		1,928
Generation | Gulf Coast | Other revenue, derivative | Operating Segments
Disaggregation of Revenue [Line Items]
Derivative revenue	(4)		1
Generation | East/West/Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	466		1,147
Contract amortization	0		0
Lease revenue	3		6
Generation | East/West/Other | Energy revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	278		735
Derivative revenue	(25)		7
Generation | East/West/Other | Capacity revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	187		487
Derivative revenue	45		110
Generation | East/West/Other | Retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers
Generation | East/West/Other | Retail revenue | Mass customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Generation | East/West/Other | Retail revenue | Business Solutions customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Generation | East/West/Other | Total retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0		0
Generation | East/West/Other | Derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	27		2
Derivative revenue	55		135
Generation | East/West/Other | Other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	32		64
Generation | East/West/Other | Total operating revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues	524		1,288
Generation | East/West/Other | Other revenue, derivative | Operating Segments
Disaggregation of Revenue [Line Items]
Lease revenue	3
Derivative revenue	$ 8		$ 16

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