Changes in Capital Structure - Changes in NRG's common shares outstanding and issued (Details) - shares		1 Months Ended	6 Months Ended
	Aug. 07, 2019	Aug. 07, 2019	Jun. 30, 2019
Schedule of Stock by Class, Equity [Roll Forward]
Balance as of beginning of period (in shares)		421,830,474	420,288,886
Treasury shares, balance as of beginning of period (in shares)		(163,259,876)	(136,638,847)
Outstanding, as of beginning of period (in shares)		258,570,598	283,650,039
Shares issued under LTIPs (in shares)			1,541,588
Shares repurchased (in shares)			(26,621,029)
Balance as of end of period (in shares)			421,830,474
Treasury shares, balance as of end of the period (in shares)			(163,259,876)
Outstanding, as of end of period (in shares)			258,570,598
Issued
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)			1,541,588
Shares repurchased (in shares)			0
Treasury
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)			0
Shares repurchased (in shares)			(26,621,029)
Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)		3,827
Shares repurchased (in shares)		(5,586,536)
Balance as of end of period (in shares)	421,834,301	421,834,301
Treasury shares, balance as of end of the period (in shares)	(168,846,412)	(168,846,412)
Outstanding, as of end of period (in shares)	252,987,889	252,987,889
Subsequent Event | Issued
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)		3,827
Shares repurchased (in shares)		0
Subsequent Event | Treasury
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)		0
Shares repurchased (in shares)		(5,586,536)
Capital Allocation Plan
Schedule of Stock by Class, Equity [Roll Forward]
Shares repurchased (in shares)			(26,621,029)
Capital Allocation Plan | Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares repurchased (in shares)	(32,207,565)