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Cover - USD ($)
 12 Months Ended
Dec. 31, 2020
Mar. 01, 2021
Jun. 30, 2020
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 001-15891    
Entity Registrant Name NRG Energy, Inc    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 41-1724239    
Entity Address, Address Line One 804 Carnegie Center    
Entity Address, City or Town Princeton    
Entity Address, State or Province NJ    
Entity Address, Postal Zip Code 08540    
City Area Code 609    
Local Phone Number 524-4500    
Title of 12(b) Security Common Stock, par value $0.01    
Trading Symbol NRG    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 6,941,658,699
Entity Common Stock, Shares Outstanding   244,687,907  
Documents Incorporated by Reference
Portions of the Registrant's definitive Proxy Statement relating to its 2021 Annual Meeting of Stockholders
are incorporated by reference into Part III of this Annual Report on Form 10-K
    
Entity Central Index Key 0001013871    
Amendment Flag false    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY