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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, 2017
Dec. 31, 2016
Dec. 31, 2015
One-percentage-point change in assumed health care cost trend rates      
Effect on total service and interest cost components, 1-Percentage-Point Increase $ 1    
Effect on total service and interest cost components, 1-Percentage-Point Decrease 0    
Effect on postretirement benefit obligation, 1-Percentage-Point Increase 9    
Effect on postretirement benefit obligation, 1-Percentage-Point Decrease (8)    
Company's contributions to 401(k) plans      
Company contributions to defined contribution plans $ 56 $ 55 $ 53
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 $ 8 7  
Expected reimbursement of contribution to retirement plan obligations to STPNOC in 2014 6    
Pension Benefit Payments      
NRG's expected future benefit payments      
2018 68    
2019 71    
2020 75    
2021 79    
2022 82    
2023-2027 421    
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP      
Funded status — STPNOC benefit plans (225) (288)  
Net periodic benefit costs 15 15 25
Total recognized in other comprehensive loss 41 (26)  
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 (76) (74)  
Net periodic benefit costs 8 7  
Total recognized in other comprehensive loss (6) 11  
Other Postretirement Benefit      
NRG's expected future benefit payments      
2018 7    
2019 8    
2020 8    
2021 8    
2022 8    
2023-2027 33    
Medicare prescription drug reimbursements      
2018 0    
2021 0    
2020 0    
2021 0    
2022 0    
2023-2027 1    
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP      
Funded status — STPNOC benefit plans (128) (128)  
Net periodic benefit costs (5) 3 $ (6)
Total recognized in other comprehensive loss (15) 38  
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 (24) (23)  
Net periodic benefit costs (3) (2)  
Total recognized in other comprehensive loss $ 5 $ (1)