| Cover | Jan. 05, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jan. 5, 2021 |
| Entity File Number | 001-15891 |
| Entity Registrant Name | NRG Energy, Inc. |
| Entity Central Index Key | 0001013871 |
| Entity Tax Identification Number | 41-1724239 |
| Entity Incorporation, State or Country Code | DE |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | NRG |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
