Summary of Significant Accounting Policies - Disaggregated Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									$ 8,148
Mark-to-market for economic hedging activities									14	$ 206	$ (128)
Operating revenues	$ 1,992	$ 2,960	$ 2,461	$ 2,065	$ 2,154	$ 2,618	$ 2,281	$ 2,021	9,478	9,074	8,915
Derivative revenue									31	195	(100)
Contigent lease revenue									104	253	272
Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									(20)
Operating revenues									(1,068)	(923)	(1,122)
Energy revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									1,548
Derivative revenue									1,284
Energy revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									(1,129)
Derivative revenue									(1,117)
Capacity revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									670
Derivative revenue									138
Capacity revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Derivative revenue									0
Retail revenue | Mass customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									5,613
Retail revenue | Mass customers | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									(5)
Retail revenue | Business Solutions customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									1,492
Retail revenue | Business Solutions customers | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Total retail revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									7,105
Total retail revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									(5)
Derivative revenue
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities									(130)
Derivative revenue									1,309
Derivative revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities									79
Derivative revenue									(1,037)
Other revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									285
Other revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									(2)
Total operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues									9,478
Total operating revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(1,057)
Lease Revenue
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									21
Lease Revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Other derivative revenue
Disaggregation of Revenue [Line Items]
Derivative revenue									16
Other derivative revenue | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Derivative revenue									0
Retail | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									7,097
Operating revenues									7,103	6,369	6,330
Retail | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(9)	(4)	(16)
Retail | Energy revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Derivative revenue									0
Retail | Capacity revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Derivative revenue									0
Retail | Retail revenue | Mass customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									5,618
Retail | Retail revenue | Business Solutions customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									1,492
Retail | Total retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									7,110
Retail | Derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities									(7)
Derivative revenue									(7)
Retail | Other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Retail | Total operating revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues									7,103
Retail | Lease Revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									13
Retail | Other derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Derivative revenue									0
Contigent lease revenue									17
Generation | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									1,071
Operating revenues									3,432	3,615	3,633
Generation | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(1,085)	$ (877)	$ (999)
Generation | Energy revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									2,677
Derivative revenue									2,401
Generation | Capacity revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									670
Derivative revenue									138
Generation | Retail revenue | Mass customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Generation | Retail revenue | Business Solutions customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Generation | Total retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Generation | Derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities									(202)
Derivative revenue									2,353
Generation | Other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									287
Generation | Total operating revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues									3,432
Generation | Lease Revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									8
Generation | Other derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Derivative revenue									16
Generation | TEXAS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									(664)
Generation | TEXAS | Energy revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									1,585
Derivative revenue									2,332
Generation | TEXAS | Capacity revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									1
Derivative revenue									0
Generation | TEXAS | Retail revenue | Mass customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Generation | TEXAS | Retail revenue | Business Solutions customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Generation | TEXAS | Total retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Generation | TEXAS | Derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities									(174)
Derivative revenue									2,160
Generation | TEXAS | Other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									84
Generation | TEXAS | Total operating revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues									1,496
Generation | TEXAS | Lease Revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Generation | TEXAS | Other derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Derivative revenue									2
Generation | East/West/Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									1,735
Generation | East/West/Other | Energy revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									1,092
Derivative revenue									69
Contigent lease revenue									5
Generation | East/West/Other | Capacity revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									669
Derivative revenue									138
Generation | East/West/Other | Retail revenue | Mass customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Generation | East/West/Other | Retail revenue | Business Solutions customers | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Generation | East/West/Other | Total retail revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									0
Generation | East/West/Other | Derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities									(28)
Derivative revenue									193
Generation | East/West/Other | Other revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									203
Generation | East/West/Other | Total operating revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues									1,936
Generation | East/West/Other | Lease Revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									8
Generation | East/West/Other | Other derivative revenue | Operating Segments
Disaggregation of Revenue [Line Items]
Derivative revenue									$ 14