Benefit Plans and Other Postretirement Benefits - Amounts Recognized in the Balance Sheets and OCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Amounts recognized in balance sheet
Other current liabilities	$ 0	$ 0
Other non-current liabilities	116	217
Amounts recognized in accumulated OCI
Net loss	52	127
Prior service cost/(credit)	2	2
Total/Net accumulated OCI	54	129
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial gain	(72)	(6)
Amortization of net actuarial loss	(1)	(5)
Amortization of prior service cost	0	0
Effect of settlement	(2)	0
Other changes in plan assets and benefit obligations recognized in other comprehensive income	(75)	(11)
Net periodic benefit credit	(27)	(8)	$ 0
Net recognized in net periodic pension credit and OCI	(102)	(19)
Significant components of NRG's domestic pension plan
Projected benefit obligation	1,452	1,489	1,397
Accumulated benefit obligation	1,423	1,455
Fair value of plan assets	1,336	1,272	1,150
Other Postretirement Benefits Plan
Amounts recognized in balance sheet
Other current liabilities	7	5
Other non-current liabilities	98	85
Amounts recognized in accumulated OCI
Net loss	5	6
Prior service cost/(credit)	(19)	(29)
Total/Net accumulated OCI	(14)	(23)
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial gain	0	0
Amortization of net actuarial loss	(1)	(1)
Amortization of prior service cost	10	14
Effect of settlement	0	0
Other changes in plan assets and benefit obligations recognized in other comprehensive income	9	13
Net periodic benefit credit	(6)	(10)	(9)
Net recognized in net periodic pension credit and OCI	3	3
Significant components of NRG's domestic pension plan
Projected benefit obligation	105	90	93
Fair value of plan assets	$ 0	$ 0	$ 0