Changes in Capital Structure - Changes in NRG's common shares outstanding and issued (Details) - shares	1 Months Ended	9 Months Ended
Oct. 31, 2019	Sep. 30, 2019
Schedule of Stock by Class, Equity [Roll Forward]
Balance as of beginning of period (in shares)	421,859,844	420,288,886
Treasury shares, balance as of beginning of period (in shares)	(169,874,327)	(136,638,847)
Outstanding, as of beginning of period (in shares)	251,985,517	283,650,039
Shares issued under LTIPs (in shares)		1,570,958
Shares repurchased (in shares)		(33,235,480)
Balance as of end of period (in shares)		421,859,844
Treasury shares, balance as of end of the period (in shares)		(169,874,327)
Outstanding, as of end of period (in shares)		251,985,517
Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	3,757
Shares repurchased (in shares)	(441,112)
Balance as of end of period (in shares)	421,863,601
Treasury shares, balance as of end of the period (in shares)	(170,269,311)
Outstanding, as of end of period (in shares)	251,594,290
Issued
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)		1,570,958
Shares repurchased (in shares)		0
Issued \| Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	3,757
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)		(33,235,480)
Treasury \| Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	0
Shares repurchased (in shares)	(441,112)
Capital Allocation Plan
Schedule of Stock by Class, Equity [Roll Forward]
Shares repurchased (in shares)		(33,235,480)
Capital Allocation Plan \| Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares repurchased (in shares)	(33,676,592)
ESPP \| Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	46,128
ESPP \| Issued \| Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	0
ESPP \| Treasury \| Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	46,128

Schedule of Stock by Class, Equity [Roll Forward]

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends.

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