Benefit Plans and Other Postretirement Benefits (Details 4) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	$ 55	$ 53	$ 47
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	$ 7	9
Expected reimbursement of contribution to retirement plan obligations to STPNOC in 2014	12
Pension Benefits
NRG's expected future benefit payments
Expected future benefit payments, Next Twelve Months	66
Expected future benefit payments, Year Two	69
Expected future benefit payments, Year Three	72
Expected future benefit payments, Year Four	76
Expected future benefit payments, Year Five	79
Expected future benefit payments, Five Fiscal Years Thereafter	417
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP
Funded status — STPNOC benefit plans	(288)	(280)
Net periodic benefit costs	15	25	15
Total recognized in other comprehensive loss	(26)	34
Pension Benefits | South Texas Project
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP
Funded status — STPNOC benefit plans	(74)	(63)
Net periodic benefit costs	7	10
Total recognized in other comprehensive loss	11	(8)
Other Postretirement Benefits
NRG's expected future benefit payments
Expected future benefit payments, Next Twelve Months	8
Expected future benefit payments, Year Two	8
Expected future benefit payments, Year Three	8
Expected future benefit payments, Year Four	9
Expected future benefit payments, Year Five	9
Expected future benefit payments, Five Fiscal Years Thereafter	38
Medicare prescription drug reimbursements
Expected Medicare prescription drug reimbursements, Next Twelve Months	0
Expected Medicare prescription drug reimbursements, Year Two	0
Expected Medicare prescription drug reimbursements, Year Three	0
Expected Medicare prescription drug reimbursements, Year Four	0
Expected Medicare prescription drug reimbursements, Year Five	0
Expected Medicare prescription drug reimbursements, After Year Five	1
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP
Funded status — STPNOC benefit plans	(128)	(178)
Net periodic benefit costs	3	(6)	$ (5)
Total recognized in other comprehensive loss	38	45
Other Postretirement Benefits | South Texas Project
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP
Funded status — STPNOC benefit plans	(23)	(26)
Net periodic benefit costs	(2)	(8)
Total recognized in other comprehensive loss	$ (1)	$ 6