Variable Interest Entities Variable Interest Entities - RPV (Details) $ in Millions	Apr. 09, 2015 generatingunit MW	Sep. 30, 2015 USD ($) MW	Dec. 31, 2014 USD ($)	[1]
Schedule of Equity Method Investments [Line Items]
Power Generation Capacity, Megawatts | MW		3,948
Equity investments in affiliates		$ 553	$ 227
NRG RPV Holdco [Member]
Schedule of Equity Method Investments [Line Items]
Equity investments in affiliates		47
Existing Portfolio of Leases [Member] | NRG RPV Holdco [Member]
Schedule of Equity Method Investments [Line Items]
Number of Solar Leases in Portfolio | generatingunit	2,200
Power Generation Capacity, Megawatts | MW	17
Remaining Lease Term	17 years
Equity investments in affiliates		26
Tax Equity Financed Portfolio of Leases [Member] | NRG RPV Holdco [Member]
Schedule of Equity Method Investments [Line Items]
Number of Solar Leases in Portfolio | generatingunit	13,000
Power Generation Capacity, Megawatts | MW	90
Equity investments in affiliates		21
Minimum [Member] | Tax Equity Financed Portfolio of Leases [Member] | NRG RPV Holdco [Member]
Schedule of Equity Method Investments [Line Items]
Remaining Lease Term	17 years
Maximum [Member] | Tax Equity Financed Portfolio of Leases [Member] | NRG RPV Holdco [Member]
Schedule of Equity Method Investments [Line Items]
Remaining Lease Term	20 years
Equity investments in affiliates		$ 150

[1] Retrospectively adjusted as discussed in Note 1, Nature of Business.