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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 |

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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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