Earnings/(Loss) Per Share - Reconciliation of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Jun. 13, 2016	May 24, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Repurchase of shares (as percent)		100.00%		100.00%
Basic and diluted loss per share attributable to NRG Energy, Inc. common stockholders
Net loss attributable to NRG Energy, Inc.	$ (626)	$ (271)	$ (790)	$ (189)
Dividends for preferred shares	0	0	0	5
Gain on redemption of preferred shares	0	(78)	0	(78)
Loss Available for Common Stockholders	$ (626)	$ (193)	$ (790)	$ (116)
Weighted average number of common shares outstanding - basic and diluted (in shares)	316	315	316	315
Loss per weighted average common share — basic and diluted (dollars per share)	$ (1.98)	$ (0.61)	$ (2.50)	$ (0.37)
Convertible Preferred Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Repurchase of shares (as percent)						100.00%	100.00%
Preferred stock, dividend rate amended (as percent)				2.822%	2.822%