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Summary of Significant Accounting Policies - Disaggregated Revenues (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Disaggregation of Revenue [Line Items]        
Total operating revenues $ 332 $ 296 $ 919 $ 797
Lease income   (252) (794) (675)
Conventional Generation        
Disaggregation of Revenue [Line Items]        
Lease income   (91) (344) (257)
Renewables        
Disaggregation of Revenue [Line Items]        
Lease income   (160) (449) (416)
Thermal        
Disaggregation of Revenue [Line Items]        
Lease income   (1) (1) (2)
Energy Revenue        
Disaggregation of Revenue [Line Items]        
Lease income   (163) (456) (422)
Energy Revenue | Conventional Generation        
Disaggregation of Revenue [Line Items]        
Lease income   (2)   (4)
Energy Revenue | Renewables        
Disaggregation of Revenue [Line Items]        
Lease income   (160)   (416)
Energy Revenue | Thermal        
Disaggregation of Revenue [Line Items]        
Lease income   (1)   (2)
Capacity Revenue        
Disaggregation of Revenue [Line Items]        
Lease income   (89) (338) (253)
Capacity Revenue | Conventional Generation        
Disaggregation of Revenue [Line Items]        
Lease income   (89)   (253)
Capacity Revenue | Renewables        
Disaggregation of Revenue [Line Items]        
Lease income   0   0
Capacity Revenue | Thermal        
Disaggregation of Revenue [Line Items]        
Lease income   0   0
Operating Segments        
Disaggregation of Revenue [Line Items]        
Total operating revenues 332 296 919 797
Contract amortization 22 18 66 52
Mark-to-market for economic hedges   (2) (8) (9)
Lease income (275) (252) (794) (675)
Total revenue from contracts with customers 79 64 199 183
Operating Segments | Conventional Generation        
Disaggregation of Revenue [Line Items]        
Total operating revenues 116 90 326 253
Contract amortization 6 1 18 4
Mark-to-market for economic hedges   0 0 0
Lease income (122) (91) (344) (257)
Total revenue from contracts with customers 0 0 0 0
Operating Segments | Renewables        
Disaggregation of Revenue [Line Items]        
Total operating revenues 157 151 444 392
Contract amortization 15 16 46 46
Mark-to-market for economic hedges   (2) (8) (9)
Lease income (153) (160) (449) (416)
Total revenue from contracts with customers 19 9 49 31
Operating Segments | Thermal        
Disaggregation of Revenue [Line Items]        
Total operating revenues 59 55 149 152
Contract amortization 1 1 2 2
Mark-to-market for economic hedges   0 0 0
Lease income 0 (1) (1) (2)
Total revenue from contracts with customers 60 55 150 152
Operating Segments | Energy Revenue        
Disaggregation of Revenue [Line Items]        
Total operating revenues 196 202 569 536
Lease income (156) (163) (456) (422)
Operating Segments | Energy Revenue | Conventional Generation        
Disaggregation of Revenue [Line Items]        
Total operating revenues 3 2 6 4
Lease income (3) (2) (6) (4)
Operating Segments | Energy Revenue | Renewables        
Disaggregation of Revenue [Line Items]        
Total operating revenues 166 167 486 441
Lease income (153) (160) (449) (416)
Operating Segments | Energy Revenue | Thermal        
Disaggregation of Revenue [Line Items]        
Total operating revenues 27 33 77 91
Lease income 0 (1) (1) (2)
Operating Segments | Capacity Revenue        
Disaggregation of Revenue [Line Items]        
Total operating revenues 143 103 388 294
Lease income (119) (89) (338) (253)
Operating Segments | Capacity Revenue | Conventional Generation        
Disaggregation of Revenue [Line Items]        
Total operating revenues 119 89 338 253
Lease income (119) (89) (338) (253)
Operating Segments | Capacity Revenue | Renewables        
Disaggregation of Revenue [Line Items]        
Total operating revenues 0 0 0 0
Lease income 0 0 0 0
Operating Segments | Capacity Revenue | Thermal        
Disaggregation of Revenue [Line Items]        
Total operating revenues 24 14 50 41
Lease income 0 0 0 0
Operating Segments | Other revenue        
Disaggregation of Revenue [Line Items]        
Total operating revenues 15 11 36 28
Operating Segments | Other revenue | Conventional Generation        
Disaggregation of Revenue [Line Items]        
Total operating revenues 0 0 0 0
Operating Segments | Other revenue | Renewables        
Disaggregation of Revenue [Line Items]        
Total operating revenues 6 2 12 6
Operating Segments | Other revenue | Thermal        
Disaggregation of Revenue [Line Items]        
Total operating revenues $ 9 $ 9 $ 24 $ 22