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Changes in Capital Structure - Changes in NRG's common shares outstanding and issued (Details) - shares
 3 Months Ended
Jan. 01, 2018
Mar. 31, 2018
Schedule of Stock by Class, Equity [Roll Forward]    
Balance as of beginning of period (in shares) 418,323,134 418,323,134
Treasury shares, balance as of beginning of period (in shares) (101,580,045) (101,580,045)
Outstanding, as of beginning of period (in shares) 316,743,089 316,743,089
Shares issued under LTIPs   1,081,994
Shares issued under ESPP 175,862  
Shares repurchased   (3,114,748)
Balance as of end of period (in shares)   419,405,128
Treasury shares, balance as of end of the period (in shares)   (104,518,931)
Outstanding, as of end of period (in shares)   314,886,197
Issued    
Schedule of Stock by Class, Equity [Roll Forward]    
Shares issued under LTIPs   1,081,994
Shares issued under ESPP 0  
Shares repurchased   0
Treasury    
Schedule of Stock by Class, Equity [Roll Forward]    
Shares issued under LTIPs   0
Shares issued under ESPP 175,862  
Shares repurchased   (3,114,748)