Benefit Plans and Other Postretirement Benefits - Amounts Recognized in the Balance Sheets and OCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Pension Benefits
Amounts recognized in balance sheet
Other current liabilities	$ 0	$ 0
Other non-current liabilities	103	134
Amounts recognized in accumulated OCI
Net loss/(gain)	50	73
Prior service credit	0	0
Total/Net accumulated OCI	50	73
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial gain	(19)	(4)
Amortization of net actuarial (gain)/loss	(4)	(2)
Settlement loss	0	6
Amortization of prior service cost	0	0
Total recognized in OCI	(23)	0
Net periodic benefit cost/(credit)	7	0	$ 21
Net recognized in net periodic pension (credit)/cost and OCI	(16)	0
Significant components of NRG's domestic pension plan
Projected benefit obligation	887	901	1,023
Accumulated benefit obligation	882	895
Fair value of plan assets	784	767	851
Other Postretirement Benefits Plan
Amounts recognized in balance sheet
Other current liabilities	4	5
Other non-current liabilities	57	65
Amounts recognized in accumulated OCI
Net loss/(gain)	(18)	(14)
Prior service credit	0	(1)
Total/Net accumulated OCI	(18)	(15)
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial gain	(4)	(1)
Amortization of net actuarial (gain)/loss	0	1
Settlement loss	0	0
Amortization of prior service cost	1	3
Total recognized in OCI	(3)	3
Net periodic benefit cost/(credit)	3	(1)	(3)
Net recognized in net periodic pension (credit)/cost and OCI	0	2
Significant components of NRG's domestic pension plan
Projected benefit obligation	61	70	75
Fair value of plan assets	$ 0	$ 0	$ 0