Changes in Capital Structure - Changes in NRG's common stock issued and outstanding (Details) - shares	1 Months Ended	3 Months Ended
May 06, 2021	Mar. 31, 2021
Schedule of Stock by Class, Equity [Roll Forward]
Balance as of beginning of period (in shares)	423,519,121	423,057,848
Treasury shares, balance as of beginning of period (in shares)	(178,825,915)	(178,825,915)
Outstanding, as of beginning of period (in shares)	244,693,206	244,231,933
Balance as of end of period (in shares)		423,519,121
Treasury shares, balance as of end of the period (in shares)		(178,825,915)
Outstanding, as of end of period (in shares)		244,693,206
Shares issued under LTIPs
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)		461,273
Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Outstanding, as of end of period (in shares)	244,753,963
Subsequent Event \| Shares issued under LTIPs
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	790
Subsequent Event \| Shares issued under ESPP
Schedule of Stock by Class, Equity [Roll Forward]
Stock issued under ESPP (in shares)	59,967
Common Stock
Schedule of Stock by Class, Equity [Roll Forward]
Balance as of beginning of period (in shares)	423,519,121	423,057,848
Balance as of end of period (in shares)		423,519,121
Common Stock \| Shares issued under LTIPs
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)		461,273
Common Stock \| Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Balance as of end of period (in shares)	423,519,911
Common Stock \| Subsequent Event \| Shares issued under LTIPs
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	790
Common Stock \| Subsequent Event \| Shares issued under ESPP
Schedule of Stock by Class, Equity [Roll Forward]
Stock issued under ESPP (in shares)	0
Treasury
Schedule of Stock by Class, Equity [Roll Forward]
Treasury shares, balance as of beginning of period (in shares)	(178,825,915)	(178,825,915)
Treasury shares, balance as of end of the period (in shares)		(178,825,915)
Treasury \| Shares issued under LTIPs
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)		0
Treasury \| Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Treasury shares, balance as of end of the period (in shares)	(178,765,948)
Treasury \| Subsequent Event \| Shares issued under LTIPs
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	0
Treasury \| Subsequent Event \| Shares issued under ESPP
Schedule of Stock by Class, Equity [Roll Forward]
Stock issued under ESPP (in shares)	59,967

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 issued during the period as a result of an employee stock purchase plan.

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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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