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Benefit Plans and Other Postretirement Benefits - Expected Future Benefit Payments For the Next Five Years and Other (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
One-percentage-point change in assumed health care cost trend rates      
Effect on postretirement benefit obligation, 1-Percentage-Point Increase $ 5    
Effect on postretirement benefit obligation, 1-Percentage-Point Decrease (4)    
Company's contributions to 401(k) plans      
Company contributions to defined contribution plans $ 28 $ 56 $ 55
South Texas Project      
STP Defined Benefit Plans      
Ownership interest in STP (as a percent) 44.00%    
Percentage of contribution to the retirement plan obligation reimbursed 44.00%    
Amount reimbursed to STPNOC towards defined benefit plans $ 13 8  
Expected reimbursement of contribution to retirement plan obligations to STPNOC in 2014 18    
Pension Benefit Payments      
NRG's expected future benefit payments      
2019 72    
2020 76    
2021 79    
2022 82    
2023 85    
2024-2028 418    
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP      
Funded status — STPNOC benefit plans (241) (225)  
Net periodic benefit costs     15
Total recognized in other comprehensive loss (37) 41  
Pension Benefit Payments | South Texas Project      
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP      
Funded status — STPNOC benefit plans (78) (76)  
Net periodic benefit costs 8 8  
Total recognized in other comprehensive loss (7) (6)  
Other Postretirement Benefit      
NRG's expected future benefit payments      
2019 7    
2020 7    
2021 7    
2022 6    
2023 6    
2024-2028 26    
Medicare prescription drug reimbursements      
2019 0    
2020 0    
2021 0    
2022 0    
2023 0    
2024-2028 1    
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP      
Funded status — STPNOC benefit plans (83) (128)  
Net periodic benefit costs     $ 3
Total recognized in other comprehensive loss 21 (15)  
Other Postretirement Benefit | South Texas Project      
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP      
Funded status — STPNOC benefit plans (19) (24)  
Net periodic benefit costs (7) (3)  
Total recognized in other comprehensive loss $ 2 $ 5