Changes in Capital Structure - Changes in NRG's common shares outstanding and issued (Details)	6 Months Ended
	Jun. 30, 2017 shares
Class of Stock [Line Items]
Balance as of beginning of period	417,583,825
Treasury shares, balance as of beginning of period	(102,140,814)
Outstanding, as of beginning of period	315,443,011
Shares issued under LTIPs	397,287
Shares issued under ESPP	282,530
Balance as of end of period	417,981,112
Treasury shares, balance as of end of the period	(101,858,284)
Outstanding, as of end of period	316,122,828
Issued
Class of Stock [Line Items]
Shares issued under LTIPs	397,287
Shares issued under ESPP	0
Treasury
Class of Stock [Line Items]
Shares issued under LTIPs	0
Shares issued under ESPP	282,530