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Changes in Capital Structure - Changes in NRG's common shares outstanding and issued (Details) - shares
 3 Months Ended
May 02, 2019
Mar. 31, 2019
Schedule of Stock by Class, Equity [Roll Forward]    
Balance as of beginning of period (in shares)   420,288,886
Treasury shares, balance as of beginning of period (in shares)   (136,638,847)
Outstanding, as of beginning of period (in shares)   283,650,039
Shares issued under LTIPs (in shares)   1,497,175
Shares repurchased (in shares)   (17,608,957)
Balance as of end of period (in shares)   421,786,061
Treasury shares, balance as of end of the period (in shares)   (154,247,804)
Outstanding, as of end of period (in shares)   267,538,257
Issued    
Schedule of Stock by Class, Equity [Roll Forward]    
Shares issued under LTIPs (in shares)   1,497,175
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)   (17,608,957)
Capital Allocation Plan    
Schedule of Stock by Class, Equity [Roll Forward]    
Shares repurchased (in shares)   (17,608,957)
Capital Allocation Plan | Subsequent Event    
Schedule of Stock by Class, Equity [Roll Forward]    
Shares repurchased (in shares) (17,999,865)