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Changes in Capital Structure - Changes in NRG's common stock issued and outstanding (Details) - shares
 1 Months Ended 9 Months Ended
Nov. 04, 2021
Sep. 30, 2021
Schedule of Stock by Class, Equity [Roll Forward]    
Balance as of beginning of period (in shares) 423,545,261 423,057,848
Treasury shares, balance as of beginning of period (in shares) (178,765,948) (178,825,915)
Outstanding, as of beginning of period (in shares) 244,779,313 244,231,933
Balance as of end of period (in shares)   423,545,261
Treasury shares, balance as of end of the period (in shares)   (178,765,948)
Outstanding, as of end of period (in shares)   244,779,313
Shares issued under LTIPs    
Schedule of Stock by Class, Equity [Roll Forward]    
Shares issued under LTIPs (in shares)   487,413
Shares issued under ESPP    
Schedule of Stock by Class, Equity [Roll Forward]    
Stock issued under ESPP (in shares)   59,967
Subsequent Event    
Schedule of Stock by Class, Equity [Roll Forward]    
Outstanding, as of end of period (in shares) 244,838,651  
Subsequent Event | Shares issued under LTIPs    
Schedule of Stock by Class, Equity [Roll Forward]    
Shares issued under LTIPs (in shares) 1,913  
Subsequent Event | Shares issued under ESPP    
Schedule of Stock by Class, Equity [Roll Forward]    
Stock issued under ESPP (in shares) 57,425  
Common Stock    
Schedule of Stock by Class, Equity [Roll Forward]    
Balance as of beginning of period (in shares) 423,545,261 423,057,848
Balance as of end of period (in shares)   423,545,261
Common Stock | Shares issued under LTIPs    
Schedule of Stock by Class, Equity [Roll Forward]    
Shares issued under LTIPs (in shares)   487,413
Common Stock | Shares issued under ESPP    
Schedule of Stock by Class, Equity [Roll Forward]    
Stock issued under ESPP (in shares)   0
Common Stock | Subsequent Event    
Schedule of Stock by Class, Equity [Roll Forward]    
Balance as of end of period (in shares) 423,547,174  
Common Stock | Subsequent Event | Shares issued under LTIPs    
Schedule of Stock by Class, Equity [Roll Forward]    
Shares issued under LTIPs (in shares) 1,913  
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) (178,765,948) (178,825,915)
Treasury shares, balance as of end of the period (in shares)   (178,765,948)
Treasury | Shares issued under LTIPs    
Schedule of Stock by Class, Equity [Roll Forward]    
Shares issued under LTIPs (in shares)   0
Treasury | Shares issued under ESPP    
Schedule of Stock by Class, Equity [Roll Forward]    
Stock issued under ESPP (in shares)   59,967
Treasury | Subsequent Event    
Schedule of Stock by Class, Equity [Roll Forward]    
Treasury shares, balance as of end of the period (in shares) (178,708,523)  
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) 57,425