Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 118	$ 25	$ 379	$ 80
Total revenue	119	118	394	370
Electricity sales
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	115	23	371	73
Total revenue	115	116	373	354
Electricity sales under PSA
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	101	17	347	55
Electricity sales to market
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	12	5	19	12
Stand-alone REC sales
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2	1	5	6
Other revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3	2	8	7
Total revenue	4	2	21	16
Related party management service fees
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3	2	8	7
Other electricity sales
Disaggregation of Revenue [Line Items]
Total revenue	0	93	2	281
Other revenue
Disaggregation of Revenue [Line Items]
Total revenue	$ 1	$ 0	$ 13	$ 9