Changes in Capital Structure - Changes in NRG's common stock issued and outstanding (Details) - shares	1 Months Ended	9 Months Ended
Nov. 05, 2020	Sep. 30, 2020
Schedule of Stock by Class, Equity [Roll Forward]
Balance as of beginning of period (in shares)	423,041,349	421,890,790
Treasury shares, balance as of beginning of period (in shares)	(178,893,929)	(172,894,601)
Outstanding, as of beginning of period (in shares)	244,147,420	248,996,189
Shares issued under LTIPs (in shares)		1,150,559
Stock issued under ESPP (in shares)		63,455
Shares repurchased (in shares)		(6,062,783)
Balance as of end of period (in shares)		423,041,349
Treasury shares, balance as of end of the period (in shares)		(178,893,929)
Outstanding, as of end of period (in shares)		244,147,420
Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	5,400
Stock issued under ESPP (in shares)	68,014
Outstanding, as of end of period (in shares)	244,220,834
Common Stock
Schedule of Stock by Class, Equity [Roll Forward]
Balance as of beginning of period (in shares)	423,041,349	421,890,790
Shares issued under LTIPs (in shares)		1,150,559
Stock issued under ESPP (in shares)		0
Shares repurchased (in shares)		0
Balance as of end of period (in shares)		423,041,349
Common Stock \| Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	5,400
Stock issued under ESPP (in shares)	0
Balance as of end of period (in shares)	423,046,749
Treasury
Schedule of Stock by Class, Equity [Roll Forward]
Treasury shares, balance as of beginning of period (in shares)	(178,893,929)	(172,894,601)
Shares issued under LTIPs (in shares)		0
Stock issued under ESPP (in shares)		63,455
Shares repurchased (in shares)		(6,062,783)
Treasury shares, balance as of end of the period (in shares)		(178,893,929)
Treasury \| Subsequent Event
Schedule of Stock by Class, Equity [Roll Forward]
Shares issued under LTIPs (in shares)	0
Stock issued under ESPP (in shares)	68,014
Treasury shares, balance as of end of the period (in shares)	(178,825,915)

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, after forfeiture, of shares or units issued under share-based payment arrangement. Excludes shares or units issued under employee stock ownership plan (ESOP).

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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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