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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	3 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
STP Defined Benefit Plans
Curtailment gain	$ 8
Increase to pension liability	$ 32
Company's contributions to 401(k) plans
Company contributions to defined contribution plans		$ 22	$ 22	$ 28
South Texas Project
STP Defined Benefit Plans
Amount reimbursed to STPNOC towards defined benefit plans		8	24
Expected reimbursement of contribution to retirement plan obligations to STPNOC in 2014		18
Pension Benefit Payments
NRG's expected future benefit payments
2021		90
2022		89
2023		87
2024		85
2025		83
2026-2030		386
STP Defined Benefit Plans
Curtailment gain		0	0	7
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP
Funded status — STPNOC benefit plans		(217)	(247)
Net periodic benefit credit		(8)	0	12
Other changes in plan assets and benefit obligations recognized in other comprehensive income/(loss)		(11)	47
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		(99)	(77)
Net periodic benefit credit		7	9
Other changes in plan assets and benefit obligations recognized in other comprehensive income/(loss)		22	(13)
Other Postretirement Benefits Plan
NRG's expected future benefit payments
2021		6
2022		6
2023		5
2024		5
2025		5
2026-2030		19
Medicare prescription drug reimbursements
2021		0
2022		0
2023		0
2024		0
2025		0
2026-2030		2
STP Defined Benefit Plans
Curtailment gain		0	0	10
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP
Funded status — STPNOC benefit plans		(90)	(93)
Net periodic benefit credit		(10)	(9)	$ (15)
Other changes in plan assets and benefit obligations recognized in other comprehensive income/(loss)		13	26
Other Postretirement Benefits Plan \| South Texas Project
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP
Funded status — STPNOC benefit plans		(20)	(20)
Net periodic benefit credit		(4)	(4)
Other changes in plan assets and benefit obligations recognized in other comprehensive income/(loss)		$ 5	$ 6
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%