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Benefit Plans and Other Postretirement Benefits - Fair Values of Pension Assets By Asset Category and Level Within the Fair Value Hierarchy (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Low      
Significant assumptions used to calculate NRG's benefit expense      
Aon Hewitt above median yield curve discount rate (in years) 6 months    
Maximum      
Significant assumptions used to calculate NRG's benefit expense      
Aon Hewitt above median yield curve discount rate (in years) 99 years    
Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 1,104 $ 953 $ 916
Significant assumptions used to calculate NRG's benefit obligations      
Discount rate 3.71% 4.26%  
Rate of compensation increase 3.00% 3.00%  
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 4.26% 4.52% 4.16%
Expected return on plan assets 6.85% 6.65% 6.36%
Rate of compensation increase 3.00% 3.00% 3.45%
Other Postretirement Benefit      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 0 $ 0 $ 0
Significant assumptions used to calculate NRG's benefit obligations      
Discount rate 3.71% 4.29%  
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 4.29% 4.55% 4.20%
Expected return on plan assets 0.00% 0.00% 0.00%
Rate of compensation increase 0.00% 0.00% 0.00%
Common/collective trust investment — U.S. equity | Pension Benefits      
Target allocations      
Target allocation of pension plan assets (as percent) 22.00%    
Common/collective trust investment — non-U.S. equity | Pension Benefits      
Target allocations      
Target allocation of pension plan assets (as percent) 14.00%    
Common/collective trust investment — non-core assets | Pension Benefits      
Target allocations      
Target allocation of pension plan assets (as percent) 19.00%    
Common/collective trust investment — fixed income | Pension Benefits      
Target allocations      
Target allocation of pension plan assets (as percent) 45.00%    
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets  
Significant Observable Inputs (Level 2) | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets  
Postretirement Benefit Obligation | Other Postretirement Benefit      
Significant assumptions used to calculate NRG's benefit expense      
Year Health care cost trend rate reaches ultimate trend rate 2025   2025
Postretirement Benefit Obligation | Before age 65 | Other Postretirement Benefit      
Significant assumptions used to calculate NRG's benefit expense      
Health care trend rate 8.20% 7.00%  
Postretirement Benefit Obligation | Age 65 and after | Other Postretirement Benefit      
Significant assumptions used to calculate NRG's benefit expense      
Health care trend rate 4.50% 5.00%  
Net Period Benefit Cost/Credit | Other Postretirement Benefit      
Significant assumptions used to calculate NRG's benefit expense      
Year Health care cost trend rate reaches ultimate trend rate 2025 2025 2023
Net Period Benefit Cost/Credit | Before age 65 | Other Postretirement Benefit      
Significant assumptions used to calculate NRG's benefit expense      
Health care trend rate 7.00% 7.25% 8.60%
Net Period Benefit Cost/Credit | Age 65 and after | Other Postretirement Benefit      
Significant assumptions used to calculate NRG's benefit expense      
Health care trend rate 5.00% 5.00% 5.00%
Other than measured at net asset value practical expedient | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 735 $ 651  
Other than measured at net asset value practical expedient | Common/collective trust investment — U.S. equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 256 283  
Other than measured at net asset value practical expedient | Common/collective trust investment — non-U.S. equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 66 71  
Other than measured at net asset value practical expedient | Common/collective trust investment — non-core assets | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 178    
Other than measured at net asset value practical expedient | Common/collective trust investment — global equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets   104  
Other than measured at net asset value practical expedient | Common/collective trust investment — fixed income | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 230 190  
Other than measured at net asset value practical expedient | Short-term investment fund | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 5 3  
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 5 3  
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — U.S. equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 0 0  
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — non-U.S. equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 0 0  
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — non-core assets | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 0    
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — global equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets   0  
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — fixed income | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 0 0  
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investment fund | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 5 3  
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 730 648  
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — U.S. equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 256 283  
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — non-U.S. equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 66 71  
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — non-core assets | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 178    
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — global equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets   104  
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — fixed income | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 230 190  
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Short-term investment fund | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 0 0  
Measured at net asset value practical expedient | Common/collective trust investment — non-U.S. equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 94 78  
Measured at net asset value practical expedient | Common/collective trust investment — fixed income | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 233 193  
Measured at net asset value practical expedient | Partnerships/joint ventures | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 42 31  
Measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — non-U.S. equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets  
Measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — fixed income | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets  
Measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Partnerships/joint ventures | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets  
Measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — non-U.S. equity | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets  
Measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — fixed income | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets  
Measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Partnerships/joint ventures | Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets