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Earnings /Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jun. 13, 2016 | May 24, 2016 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Redemption of shares (as percentage) | 100.00% | 100.00% |  |  |  |  |  |  |  |  |  |  |
Basic and diluted loss per share attributable to NRG common stockholders |  |  |  |  |  |  |  |  |  |  |  |  |
Net loss attributable to NRG Energy, Inc. |  |  | $ (1,535) | $ 171 | $ (626) | $ (163) | $ (987) | $ 402 | $ (271) | $ 82 | $ (2,153) | $ (774) | $ (6,382)
Dividends for preferred shares |  |  |  |  |  |  |  |  |  |  | 0 | 5 | 20
Gain on redemption of preferred shares |  |  |  |  |  |  |  |  |  |  | 0 | (78) | 0
(Loss)/Income Available for Common Stockholders |  |  | $ (1,535) | $ 171 | $ (626) | $ (163) | $ (987) | $ 402 | $ (193) | $ 77 | $ (2,153) | $ (701) | $ (6,402)
Weighted average number of common shares outstanding (in shares) |  |  |  |  |  |  |  |  |  |  | 317 | 316 | 329
Loss per weighted average common share — basic and diluted (in USD PER SHARE) |  |  |  |  |  |  |  |  |  |  | $ (6.79) | $ (2.22) | $ (19.46)
Convertible Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, dividend rate (as percent) |  | 2.822% |  |  |  |  |  |  |  |  | 2.822% | 2.822% | 2.822%
Basic and diluted loss per share attributable to NRG common stockholders |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on redemption of preferred shares | $ (78) |  |  |  |  |  |  |  |  |  |  |  |

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