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Benefit Plans and Other Postretirement Benefits (Tables)

12 Months Ended

Dec. 31, 2020

Retirement Benefits [Abstract]

Schedule of Net Benefit Cost (Credit) Related to Pension and Other Postretirement Benefit Plans Components

The annual net periodic benefit cost/(credit) related to NRG's pension and other postretirement benefit plans include the following components:

			Year Ended December 31,
			Pension Benefits
(In millions)			2020						2019						2018
Service cost benefits earned			$	10					$	10					$	23
Interest cost on benefit obligation			38						46						44
Expected return on plan assets			(61)						(59)						(62)
Amortization of unrecognized net loss			5						3						—
Settlement/curtailment expense			—						—						7
Net periodic benefit (credit)/cost			$	(8)					$	—					$	12

			Year Ended December 31,
			Other Postretirement Benefits
(In millions)			2020						2019						2018
Service cost benefits earned			$	—					$	1					$	1
Interest cost on benefit obligation			3						3						4
Amortization of unrecognized prior service cost			(14)						(13)						(10)
Amortization of unrecognized net loss			1						—						—
Curtailment gain			—						—						(10)
Net periodic benefit credit			$	(10)					$	(9)					$	(15)

Schedule of Comparison of Pension Benefit obligation, Other Postretirement Benefit Obligations and Related Plan Assets on a Combined Basis

A comparison of the pension benefit obligation, other postretirement benefit obligations and related plan assets for NRG's plans on a combined basis is as follows:

			As of December 31,
			Pension Benefits												Other Postretirement Benefits
(In millions)			2020						2019						2020						2019
Benefit obligation at January 1			$	1,397					$	1,222					$	93					$	83
Service cost			10						10						—						1
Interest cost			38						46						3						3
Plan amendments			—						—						—						(2)
Actuarial loss			126						207						—						16
Employee and retiree contributions			—						—						3						4
Benefit payments			(82)						(88)						(9)						(12)
Benefit obligation at December 31			1,489						1,397						90						93
Fair value of plan assets at January 1			1,150						981						—						—
Actual return on plan assets			193						216						—						—
Employee and retiree contributions			—						—						3						4
Employer contributions			11						41						6						7
Benefit payments			(82)						(88)						(9)						(11)
Fair value of plan assets at December 31			1,272						1,150						—						—
Funded status at December 31 — excess of obligation over assets			$	(217)					$	(247)					$	(90)					$	(93)

Schedule of Amounts Recognized in Balance Sheet

Amounts recognized in NRG's balance sheets were as follows:

			As of December 31,
			Pension Benefits												Other Postretirement Benefits
(In millions)			2020						2019						2020						2019
Other current liabilities			$	—					$	—					$	5					$	7
Other non-current liabilities			217						247						85						86

Schedule of Amounts Recognized in OCI Not Yet Recognized as Components of Net Periodic Benefit Costs

Amounts recognized in NRG's accumulated OCI that have not yet been recognized as components of net periodic benefit cost were as follows:

			As of December 31,
			Pension Benefits												Other Postretirement Benefits
(In millions)			2020						2019						2020						2019
Net loss			$	127					$	138					$	6					$	7
Prior service cost/(credit)			2						2						(29)						(43)
Total accumulated OCI			$	129					$	140					$	(23)					$	(36)

Schedule of Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income

Other changes in plan assets and benefit obligations recognized in OCI were as follows:

			Year Ended December 31,
			Pension Benefits												Other Postretirement Benefits
(In millions)			2020						2019						2020						2019
Net actuarial (gain)/loss			$	(6)					$	50					$	—					$	16
Amortization of net actuarial (gain)			(5)						(3)						(1)						—
Prior service credit			—						—						—						(2)
Amortization of prior service cost			—						—						14						12
Total recognized in OCI			$	(11)					$	47					$	13					$	26
Net periodic benefit credit			(8)						—						(10)						(9)
Net recognized in net periodic pension credit and OCI			$	(19)					$	47					$	3					$	17

Schedule of Benefit Obligations Significant Components

The following table presents the balances of significant components of NRG's pension plan:

			As of December 31,
			Pension Benefits
(In millions)			2020						2019
Projected benefit obligation			$	1,489					$	1,397
Accumulated benefit obligation			1,455						1,362
Fair value of plan assets			1,272						1,150

Schedule of Fair Value of Pension Plan Assets by Asset Category and Level within the Fair Value Hierarchy

NRG's market-related value of its plan assets is the fair value of the assets. The fair values of the Company's pension plan assets by asset category and their level within the fair value hierarchy are as follows:

			Fair Value Measurements as of December 31, 2020
(In millions)			Quoted Prices in Active Markets for Identical Assets (Level 1)						Significant Observable Inputs (Level 2)						Total
Common/collective trust investment — U.S. equity			$	—					$	284					$	284
Common/collective trust investment — non-U.S. equity			—						113						113
Common/collective trust investment — non-core assets			—						151						151
Common/collective trust investment — fixed income			—						258						258
Short-term investment fund			13						—						13
Subtotal fair value			$	13					$	806					$	819
Measured at net asset value practical expedient:
Common/collective trust investment — non-U.S. equity															45
Common/collective trust investment — fixed income															289
Common/collective trust investment — non-core assets															84
Partnerships/joint ventures															35
Total fair value															$	1,272

			Fair Value Measurements as of December 31, 2019
(In millions)			Quoted Prices in Active Markets for Identical Assets (Level 1)						Significant Observable Inputs (Level 2)						Total
Common/collective trust investment — U.S. equity			$	—					$	233					$	233
Common/collective trust investment — non-U.S. equity			—						73						73
Common/collective trust investment — non-core assets			—						143						143
Common/collective trust investment — fixed income			—						272						272
Short-term investment fund			12						—						12
Subtotal fair value			$	12					$	721					$	733
Measured at net asset value practical expedient:
Common/collective trust investment — non-U.S. equity															84
Common/collective trust investment — fixed income															279
Common/collective trust investment — non-core assets															24
Partnerships/joint ventures															30
Total fair value															$	1,150

Schedule of Assumptions Used to Calculate Benefit Expense

The following table presents the significant assumptions used to calculate NRG's benefit obligations:

			As of December 31,
			Pension Benefits												Other Postretirement Benefits
Weighted-Average Assumptions			2020						2019						2020						2019
Discount rate			2.56		%				3.26		%				2.54		%				3.26		%
Interest crediting rate			3.12		%				—						1.62		%				—
Rate of compensation increase			3.00		%				3.00		%				—		%				—		%
Health care trend rate			—						—						7.2% grading to 4.5% in 2028						7.5% grading to 4.5% in 2028

The following table presents the significant assumptions used to calculate NRG's benefit expense:

			As of December 31,
			Pension Benefits																		Other Postretirement Benefits
Weighted-Average Assumptions			2020						2019						2018						2020						2019						2018
Discount rate			3.26		%				4.38%/4.20%						3.71%/4.04%						3.26%						4.37%						3.71% /4.08%
Interest crediting rate			3.66		%				—						—						2.28		%				—						—
Expected return on plan assets			5.93		%				6.35		%				6.17		%				—						—						—
Rate of compensation increase			3.00		%				3.00		%				3.00		%				—						—						—
Health care trend rate			—						—						—						7.5% grading to 4.5% in 2028						7.8% grading to 4.5% in 2025						8.2% grading to 4.5% in 2025

Schedule of Target Allocation of Pension Plan Assets

The target allocations of NRG's pension plan assets were as follows for the year ended December 31, 2020:

U.S. equity			20		%
Non-U.S. equity			13		%
Non-core assets			17		%
U.S. fixed income			50		%

Schedule of Performance Benchmarks

Performance benchmarks are composed of the following indices:

Asset Class						Index
U.S. equities						Dow Jones U.S. Total Stock Market Index
Non-U.S. equities						MSCI All Country World Ex-U.S. IMI Index
Non-core assets(a)						Various (per underlying asset class)
Fixed income securities						Barclays Short, Intermediate and Long Credits/Barclays Strips 20+ Index

(a)Non-Core Assets are defined as diversifying asset classes approved by the Investment Committee that are intended to enhance returns and/or reduce volatility of the U.S. and non-U.S. equities. Asset classes considered Non-Core include, but may not be limited to: Emerging Market Equity, Emerging Market Debt, Non-US Developed Market Small Cap, High Yield Fixed Income, Real Estate, Bank Loans, Global Infrastructure and other Alternatives.

Schedule of Expected Benefit Payments

NRG's expected future benefit payments for each of the next five years, and in the aggregate for the five years thereafter, are as follows:

									Other Postretirement Benefit
(In millions)			Pension Benefit Payments						Benefit Payments						Medicare Prescription Drug Reimbursements
2021			$	90					$	6					$	—
2022			89						6						—
2023			87						5						—
2024			85						5						—
2025			83						5						—
2026-2030			386						19						2

Schedule of Benefit Costs and Other Changes Recognized in the Financial Statements Related to its Interest in STP

The Company has recognized the following in its statement of financial position, statement of operations and accumulated OCI related to its 44% interest in STP:

			As of December 31,
			Pension Benefits												Other Postretirement Benefits
(In millions)			2020						2019						2020						2019
Funded status — STPNOC benefit plans			$	(99)					$	(77)					$	(20)					$	(20)
Net periodic benefit cost/(credit)			7						9						(4)						(4)
Other changes in plan assets and benefit obligations recognized in other comprehensive income/(loss)			22						(13)						5						6

Defined Contribution Plan Contributions

The Company's contributions to these plans were as follows:

			Year Ended December 31,
(In millions)			2020						2019						2018
Company contributions to defined contribution plans			$	22					$	22					$	28

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