Genon Mid-atlantic 10-Q 8 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015
Document and Entity Information [Line Items]
Entity Registrant Name	GenOn Energy, Inc.
Entity Central Index Key	1126294
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Document Type	10-Q
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Amendment Flag	FALSE
Entity Common Stock, Shares Outstanding	1
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	Yes
Entity Current Reporting Status	Yes
GenOn Americas Generation
Document and Entity Information [Line Items]
Entity Registrant Name	GenOn Americas Generation, LCC
Entity Central Index Key	1140761
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Entity Common Stock, Shares Outstanding	0
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	Yes
Entity Current Reporting Status	Yes
GenOn Mid-Atlantic
Document and Entity Information [Line Items]
Entity Registrant Name	GenOn Mid-Atlantic, LLC
Entity Central Index Key	1138258
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Entity Common Stock, Shares Outstanding	0
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	Yes
Entity Current Reporting Status	Yes

CONDENSED_CONSOLIDATED_STATEME

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating Revenues
Total operating revenues	$754	$1,050
Operating Costs and Expenses
Depreciation and amortization	56	62
Acquisition-related transaction and integration costs	0	1
Total operating costs and expenses	716	991
Gain (loss) on sale of assets	0	-6
Operating Income/(Loss)	38	53
Other Income/(Expense)
Other income/(expense), net	3	1
Total other income (expense), net	-50	-49
Income/(Loss) Before Income Taxes	-12	4
Income tax expense/(benefit)	-1	1
Net Income/(Loss)	-11	3
GenOn Americas Generation
Operating Revenues
Total operating revenues	726	1,065
Operating Costs and Expenses
Depreciation and amortization	17	22
Total operating costs and expenses	715	975
Gain (loss) on sale of assets	0	-6
Operating Income/(Loss)	11	84
Other Income/(Expense)
Total other income (expense), net	-18	-19
Income/(Loss) Before Income Taxes	-7	65
Income tax expense/(benefit)	0	0
Net Income/(Loss)	-7	65
GenOn Mid-Atlantic
Operating Revenues
Total operating revenues	276	327
Operating Costs and Expenses
Depreciation and amortization	16	19
Total operating costs and expenses	252	283
Operating Income/(Loss)	24	44
Other Income/(Expense)
Total other income (expense), net	-1	-1
Income/(Loss) Before Income Taxes	23	43
Income tax expense/(benefit)	0	0
Net Income/(Loss)	23	43
Non-affiliate
Operating Revenues
Total operating revenues	749	1,026
Operating Costs and Expenses
Cost of operations	524	622
Selling, general and administrative	0	18
Other Income/(Expense)
Interest expense	50	47
Non-affiliate | GenOn Americas Generation
Operating Revenues
Total operating revenues	703	957
Operating Costs and Expenses
Cost of operations	323	371
Selling, general and administrative	0	2
Other Income/(Expense)
Interest expense	16	17
Non-affiliate | GenOn Mid-Atlantic
Operating Revenues
Total operating revenues	5	-151
Operating Costs and Expenses
Cost of operations	164	234
Affiliate
Operating Revenues
Total operating revenues	5	24
Operating Costs and Expenses
Cost of operations	89	255
Selling, general and administrative	47	33
Other Income/(Expense)
Interest expense	3	3
Affiliate | GenOn Americas Generation
Operating Revenues
Total operating revenues	23	108
Operating Costs and Expenses
Cost of operations	354	562
Selling, general and administrative	21	18
Other Income/(Expense)
Interest expense	2	2
Affiliate | GenOn Mid-Atlantic
Operating Revenues
Total operating revenues	271	478
Operating Costs and Expenses
Cost of operations	57	15
Selling, general and administrative	15	15
Other Income/(Expense)
Interest expense	$1	$1

CONDENSED_CONSOLIDATED_STATEME1

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME/(LOSS) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Net (Loss)/Income	($11)	$3
Other comprehensive income net of reclassifications, net of tax of $0:
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	-1	0
Other comprehensive income/(loss)	-1
Comprehensive income/(loss)	($12)	$3

CONDENSED_CONSOLIDATED_STATEME2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME/(LOSS) Parenthetical (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	$0	$0

CONDENSED_CONSOLIDATED_BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Current Assets
Cash and cash equivalents	$1,125	$920
Funds deposited by counterparties	29	54
Accounts receivable — trade	95	120
Inventory	400	507
Cash collateral paid in support of energy risk management activities	100	38
Disposal Group, Including Discontinued Operation, Other Assets, Current	48	0
Prepayments and other current assets	138	150
Total current assets	2,433	2,391
Property, Plant and Equipment
Property, plant and equipment	3,036	3,045
Other Assets
Intangible assets, net of accumulated amortization	70	72
Disposal Group, Including Discontinued Operation, Other Assets, Noncurrent	17	17
Other non-current assets	173	184
Assets, Noncurrent	506	478
Total Assets	5,975	5,914
Current Liabilities
Current portion of long-term debt and capital leases	8	10
Cash collateral received in support of energy risk management activities	29	54
Other Accrued Liabilities, Current	252	238
Total current liabilities	856	868
Other Liabilities
Long-term debt and capital leases	3,105	3,120
Pension and Other Postretirement and Postemployment Benefit Plans, Liabilities, Noncurrent	48	0
Out-of-market contracts	951	969
Other non-current liabilities	487	484
Total non-current liabilities	4,730	4,645
Total Liabilities	5,586	5,513
Stockholder's Equity
Common stock: $0.001 par value, 1 share authorized and issued at June 30, 2013 and December 31, 2012	0	0
Additional paid-in capital	325	325
Accumulated deficit	67	78
Accumulated other comprehensive income	-3	-2
Total Stockholder's Equity	389	401
Total Liabilities and Stockholder's Equity	5,975	5,914
GenOn Mid-Atlantic
Current Assets
Cash and cash equivalents	275	157
Inventory	152	166
Prepayments and other current assets	76	80
Total current assets	733	654
Property, Plant and Equipment
Property, plant and equipment	947	958
Other Assets
Intangible assets, net of accumulated amortization	13	10
Other non-current assets	68	87
Assets, Noncurrent	231	238
Total Assets	1,911	1,850
Current Liabilities
Current portion of long-term debt and capital leases	3	5
Other Accrued Liabilities, Current	69	53
Total current liabilities	254	227
Other Liabilities
Out-of-market contracts	540	547
Other non-current liabilities	52	60
Total non-current liabilities	640	629
Total Liabilities	894	856
Stockholder's Equity
Total Liabilities and Stockholder's Equity	1,911	1,850
Member's Equity
Members' interest	1,017	994
GenOn Americas Generation
Current Assets
Cash and cash equivalents	244	103
Funds deposited by counterparties	29	54
Accounts receivable — trade	81	106
Due from Affiliate, Current	35	0
Inventory	246	318
Cash collateral paid in support of energy risk management activities	91	29
Prepayments and other current assets	85	90
Total current assets	1,897	1,883
Property, Plant and Equipment
Property, plant and equipment	1,111	1,110
Other Assets
Intangible assets, net of accumulated amortization	70	72
Other non-current assets	93	111
Assets, Noncurrent	498	439
Total Assets	3,506	3,432
Current Liabilities
Current portion of long-term debt and capital leases	3	5
Cash collateral received in support of energy risk management activities	29	54
Other Accrued Liabilities, Current	121	93
Total current liabilities	892	899
Other Liabilities
Long-term debt and capital leases	926	929
Out-of-market contracts	541	547
Other non-current liabilities	107	106
Total non-current liabilities	1,805	1,717
Total Liabilities	2,697	2,616
Commitments and Contingencies
Stockholder's Equity
Total Liabilities and Stockholder's Equity	3,506	3,432
Member's Equity
Members' interest	809	816
Non-affiliate
Current Assets
Derivative instruments	484	591
Other Assets
Derivative instruments	234	195
Current Liabilities
Accounts payable	125	135
Derivative instruments	393	382
Other Liabilities
Derivative instruments	137	69
Non-affiliate | GenOn Mid-Atlantic
Current Assets
Accounts receivable — trade	2	10
Derivative instruments	83	100
Current Liabilities
Accounts payable	24	27
Derivative instruments	1	1
Non-affiliate | GenOn Americas Generation
Current Assets
Derivative instruments	484	591
Other Assets
Derivative instruments	234	196
Current Liabilities
Accounts payable	66	50
Derivative instruments	393	382
Other Liabilities
Derivative instruments	137	69
Affiliate
Current Assets
Derivative instruments	14	11
Other Assets
Derivative instruments	12	10
Current Liabilities
Accounts payable	11	14
Derivative instruments	38	35
Other Liabilities
Derivative instruments	2	3
Affiliate | GenOn Mid-Atlantic
Current Assets
Derivative instruments	145	141
Other Assets
Derivative instruments	150	141
Current Liabilities
Accounts payable	10	14
Derivative instruments	147	127
Other Liabilities
Derivative instruments	48	22
Affiliate | GenOn Americas Generation
Current Assets
Notes Receivable, Related Parties, Current	331	331
Derivative instruments	271	261
Other Assets
Derivative instruments	101	60
Current Liabilities
Accounts payable	0	23
Derivative instruments	280	292
Other Liabilities
Derivative instruments	$94	$66

CONDENSED_CONSOLIDATED_BALANCE1

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, except Share data, unless otherwise specified
Property, plant and equipment, accumulated depreciation	$491	$436
Common stock, par value (in dollars per share)	$0.00	$0.00
Common stock, shares authorized (in shares)	1	1
Common stock, shares issued (in shares)	1	1
Intangible assets, accumulated amortization	76	66
GenOn Americas Generation
Property, plant and equipment, accumulated depreciation	188	170
Intangible assets, accumulated amortization	76	66
GenOn Mid-Atlantic
Property, plant and equipment, accumulated depreciation	151	135
Intangible assets, accumulated amortization	$45	$0

CONDENSED_CONSOLIDATED_STATEME3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Cash Flows from Operating Activities
Net (Loss)/Income	($11)	$3
Adjustments to reconcile net (loss)/income to net cash provided by operating activities:
Depreciation and amortization	56	62
Amortization of Financing Costs and Discounts	-15	-14
Amortization of out-of-market contracts and emission allowances	-3	-28
Loss on sale of asset	0	6
Changes in derivative instruments	144	237
Changes in collateral deposits supporting energy risk management activities	62	310
Changes in other working capital	141	171
Net Cash Provided by Operating Activities	250	127
Cash Flows from Investing Activities:
Capital expenditures	-44	-33
Proceeds from sale of assets, net of cash disposed of	0	50
Payments for (Proceeds from) Other Investing Activities	0	5
Net Cash (Used)/Provided by Investing Activities	-44	22
Cash Flows from Financing Activities
Payments for short and long-term debt	-1	0
Net Cash Used by Financing Activities	-1
Net Increase in Cash and Cash Equivalents	205	149
Cash and Cash Equivalents at Beginning of Period	920	760
Cash and Cash Equivalents at Beginning of Period	1,125	909
GenOn Americas Generation
Cash Flows from Operating Activities
Net (Loss)/Income	-7	65
Adjustments to reconcile net (loss)/income to net cash provided by operating activities:
Depreciation and amortization	17	22
Amortization of Debt Discount (Premium)	-2	-2
Amortization of out-of-market contracts and emission allowances	9	2
Loss on sale of asset	0	6
Changes in derivative instruments	113	149
Changes in collateral deposits supporting energy risk management activities	62	367
Changes in other working capital	89	244
Net Cash Provided by Operating Activities	157	119
Cash Flows from Investing Activities:
Capital expenditures	-15	-7
Proceeds from sale of assets, net of cash disposed of	0	50
Net Cash (Used)/Provided by Investing Activities	-15	153
Cash Flows from Financing Activities
Payments for short and long-term debt	-1	0
Net Cash Used by Financing Activities	-1	0
Net Increase in Cash and Cash Equivalents	141	272
Cash and Cash Equivalents at Beginning of Period	103	63
Cash and Cash Equivalents at Beginning of Period	244	335
GenOn Mid-Atlantic
Cash Flows from Operating Activities
Net (Loss)/Income	23	43
Adjustments to reconcile net (loss)/income to net cash provided by operating activities:
Depreciation and amortization	16	19
Amortization of out-of-market contracts and emission allowances	17	0
Changes in derivative instruments	50	155
Changes in other working capital	21	62
Net Cash Provided by Operating Activities	127	279
Cash Flows from Investing Activities:
Capital expenditures	-8	-6
Net Cash (Used)/Provided by Investing Activities	-8	-6
Cash Flows from Financing Activities
Payments for short and long-term debt	-1	0
Net Cash Used by Financing Activities	-1
Net Increase in Cash and Cash Equivalents	118	273
Cash and Cash Equivalents at Beginning of Period	157	64
Cash and Cash Equivalents at Beginning of Period	275	337
Affiliate | GenOn Americas Generation
Cash Flows from Investing Activities:
Decrease in note receivable — affiliate	$0	$110

Basis_of_Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation (GenOn, GenOn Americas Generation, GenOn Mid-Atlantic)	Basis of Presentation (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	GenOn Energy, Inc., a wholly owned subsidiary of NRG, is a wholesale power generator engaged in the ownership and operation of power generation facilities, with approximately 18,430 MW of net electric generating capacity located in the U.S. In the first quarter of 2015, the GenOn mothballed Osceola and retired Coolwater resulting in a net decrease in operating generation capacity of 1,099 MW from December 31, 2014.
	GenOn Americas Generation is a wholesale power generator with approximately 7,596 MW of net electric generating capacity located, in many cases, near major metropolitan areas. GenOn Americas Generation's electric generating capacity is part of the 18,430 MW of net electric generating capacity of GenOn.
	GenOn Mid-Atlantic operates and owns or leases 4,683 MW of net electric generating capacity in Maryland near Washington, D.C. GenOn Mid-Atlantic’s electric generating capacity is part of the 7,596 MW of net electric generating capacity of GenOn Americas Generation. GenOn Mid-Atlantic’s generating facilities serve the Eastern PJM markets.
	GenOn Americas Generation and GenOn Mid-Atlantic are Delaware limited liability companies and indirect wholly owned subsidiaries of GenOn. GenOn Mid-Atlantic is an indirect wholly owned subsidiary of GenOn Americas Generation.
	The Registrants sell power from their generation portfolio, offer capacity or similar products to retail electric providers and others, and provide Ancillary Services to support system reliability.
	On December 14, 2012, NRG completed the acquisition of GenOn, referred to herein as the NRG Merger.
	This is a combined quarterly report of the Registrants for the quarter ended March 31, 2015. The notes to the condensed consolidated financial statements apply to the Registrants as indicated parenthetically next to each corresponding disclosure. The accompanying unaudited interim condensed consolidated financial statements have been prepared in accordance with the SEC's regulations for interim financial information and with the instructions to Form 10-Q. Accordingly, they do not include all of the information and notes required by U.S. GAAP for complete financial statements. The following notes should be read in conjunction with the accounting policies and other disclosures as set forth in the notes to the Registrants' financial statements set forth in the Registrants' 2014 Form 10-K. Interim results are not necessarily indicative of results for a full year.
	In the opinion of management, the accompanying unaudited interim condensed consolidated financial statements contain all material adjustments consisting of normal and recurring accruals necessary to present fairly the Registrants' consolidated financial positions as of March 31, 2015, and the results of operations, comprehensive income/(loss) and cash flows for the three months ended March 31, 2015, and 2014.
	Use of Estimates
	The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities at the date of the financial statements, disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from these estimates.
	Reclassifications
	Certain prior year amounts have been reclassified for comparative purposes.  The reclassification did not affect results from operations.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2015
Significant Accounting Policies [Abstract]
Summary of Significant Accounting Policies (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)	Summary of Significant Accounting Policies (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	Recent Accounting Developments
	ASU 2015-02 — In February 2015, the FASB issued ASU No. 2015-02, Consolidation (Topic 810): Amendments to the Consolidation Analysis, or ASU No. 2015-02. The amendments of ASU No. 2015-02 were issued in an effort to minimize situations under previously existing guidance in which a reporting entity was required to consolidate another legal entity in which that reporting entity did not have: (1) the ability through contractual rights to act primarily on its own behalf; (2) ownership of the majority of the legal entity's voting rights; or (3) the exposure to a majority of the legal entity's economic benefits. ASU No. 2015-02 affects reporting entities that are required to evaluate whether they should consolidate certain legal entities. All legal entities are subject to reevaluation under the revised consolidation model. The guidance in ASU No. 2015-02 is effective for periods beginning after December 15, 2015. Early adoption is permitted. The Registrants adopted the standard effective January 1, 2015 and the adoption of this standard did not impact the Registrants' results of operations, cash flows or financial position.
	ASU 2014-16 — In November 2014, the FASB issued ASU No. 2014-16, Derivatives and Hedging (Topic 815): Determining Whether the Host Contract in a Hybrid Financial Instrument Issued in the Form of a Share Is More Akin to Debt or to Equity, or ASU No. 2014-16. The amendments of ASU No. 2014-16 clarify how U.S. GAAP should be applied in determining whether the nature of a host contract is more akin to debt or equity and in evaluating whether the economic characteristics and risks of an embedded feature are "clearly and closely related" to its host contract. The guidance in ASU No. 2014-16 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2015. Early adoption is permitted. The Registrants adopted the standard effective January 1, 2015 and the adoption of this standard did not impact the Registrants' results of operations, cash flows or financial position.
	ASU 2014-09 — In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), or ASU No. 2014-09. The amendments of ASU No. 2014-09 complete the joint effort between the FASB and the International Accounting Standards Board, or IASB, to develop a common revenue standard for U.S. GAAP and International Financial Reporting Standards, or IFRS, and to improve financial reporting. The guidance in ASU No. 2014-09 provides that an entity should recognize revenue to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled to in exchange for the goods or services provided and establishes the following steps to be applied by an entity: (1) identify the contract with a customer; (2) identify the performance obligations in the contract; (3) determine the transaction price; (4) allocate the transaction price to the performance obligations in the contract; and (5) recognize revenue when (or as) the entity satisfies the performance obligation. The guidance of ASU No. 2014-09 is effective for annual reporting periods beginning after December 15, 2016, including interim periods therein. Early adoption is not permitted. The Registrants are currently evaluating the impact of the standard on the Registrants' results of operations, cash flows and financial position.
	Oil Inventory Adjustment (GenOn and GenOn Americas Generation)
	During the first quarter of 2015, certain oil inventory was identified as unusable and the related value of $11 million was written off to cost of operations in the statement of operations.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments (GenOn, GenOn Americas Generation, GenOn Mid-Atlantic)	Fair Value of Financial Instruments (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	This footnote should be read in conjunction with the complete description under Note 4, Fair Value of Financial Instruments, to the Registrants' 2014 Form 10-K.
	For cash and cash equivalents, funds deposited by counterparties, accounts receivable, note receivable — affiliate, accounts payable, accrued liabilities, restricted cash, and cash collateral paid and received in support of energy risk management activities, the carrying amount approximates fair value because of the short-term maturity of those instruments and are classified as Level 1 within the fair value hierarchy.
	The estimated carrying amounts and fair values of GenOn and GenOn Americas Generation’s debt are as follows:
	GenOn
		As of March 31, 2015						As of December 31, 2014
		Carrying Amount			Fair Value			Carrying Amount		Fair Value
		(In millions)
	Long and short-term debt	$	3,107		$	2,829		$	3,122	$	2,706
	The fair value of long and short-term debt that is estimated using reported market prices for instruments that are publicly traded is classified as Level 2 within the fair value hierarchy. The fair value of non-publicly traded debt is based on the income approach valuation technique using current interest rates for similar instruments with equivalent credit quality and is classified as Level 3 within the fair value hierarchy.
	GenOn Americas Generation
		As of March 31, 2015						As of December 31, 2014
		Carrying Amount			Fair Value			Carrying Amount		Fair Value
		(In millions)
	Long and short-term debt	$	926		$	796		$	929	$	720
	The fair value of long and short-term debt is estimated using reported market prices for instruments that are publicly traded and is classified as Level 2 within the fair value hierarchy.
	Recurring Fair Value Measurements
	Derivative assets and liabilities are carried at fair market value. Realized and unrealized gains and losses included in earnings that are related to energy derivatives are recorded in operating revenues and cost of operations.
	GenOn
	The following tables present assets and liabilities (including affiliate amounts) measured and recorded at fair value on GenOn’s consolidated balance sheet on a recurring basis and their level within the fair value hierarchy:
		As of March 31, 2015
		Fair Value
		Level 1 (a)			Level 2 (a)			Level 3		Total
		(In millions)
	Derivative assets:
	Commodity contracts	$	197		$	516		$	31	$	744
	Derivative liabilities:
	Commodity contracts	$	118		$	437		$	15	$	570
	Other assets (b)	$	20		$	—		$	—	$	20
	(a) There were no transfers during the three months ended March 31, 2015, between Levels 1 and 2.
	(b) Relates to mutual funds held in a rabbi trust for non-qualified deferred compensation plans for certain key and highly compensated employees.
		As of December 31, 2014
		Fair Value
		Level 1 (a)			Level 2 (a)			Level 3		Total
		(In millions)
	Derivative assets:
	Commodity contracts	$	179		$	582		$	46	$	807
	Derivative liabilities:
	Commodity contracts	$	105		$	371		$	13	$	489
	Other assets (b)	$	21		$	—		$	—	$	21
	(a) There were no transfers during the year ended December 31, 2014, between Levels 1 and 2.
	(b) Relates to mutual funds held in a rabbi trust for non-qualified deferred compensation plans for certain key and highly compensated employees.
	The following table reconciles, for the three months ended March 31, 2015, and 2014, the beginning and ending balances for derivatives that are recognized at fair value in GenOn's consolidated financial statements at least annually using significant unobservable inputs:
		Fair Value Measurement Using Significant Unobservable Inputs (Level 3)
		Three months ended March 31,
		2015			2014
		Derivatives (a)
		(In millions)
	Beginning balance	$	33		$	(4	)
	Total (losses)/gains included in earnings — realized/unrealized	(13		)	4
	Purchases	(4		)	—
	Transfers into Level 3 (b)	—			(2		)
	Ending balance	$	16		$	(2	)
	Losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of March 31	$	(7	)	$	—
	(a) Consists of derivative assets and liabilities, net.
	(b) Transfers in/out of level 3 are related to the availability of external broker quotes and are valued as of the end of the reporting period.
	GenOn Americas Generation
	The following tables present assets and liabilities (including affiliate amounts) measured and recorded at fair value on GenOn Americas Generation's consolidated balance sheet on a recurring basis and their level within the fair value hierarchy:
		As of March 31, 2015
		Fair Value
		Level 1 (a)			Level 2 (a)			Level 3		Total
		(In millions)
	Derivative assets:
	Commodity contracts	$	235		$	816		$	39	$	1,090
Derivative liabilities:
Commodity contracts	$	146		$	735		$	23	$	904
(a) There were no transfers during the three months ended March 31, 2015, between Levels 1 and 2.
	As of December 31, 2014
	Fair Value
	Level 1 (a)			Level 2 (a)			Level 3		Total
	(In millions)
Derivative assets:
Commodity contracts	$	208		$	848		$	52	$	1,108
Derivative liabilities:
Commodity contracts	$	137		$	640		$	32	$	809
(a) There were no transfers during the year ended December 31, 2014, between Levels 1 and 2.
The following table reconciles, for the three months ended March 31, 2015, and 2014, the beginning and ending balances for GenOn Americas Generation's derivatives that are recognized at fair value in the consolidated financial statements at least annually using significant unobservable inputs:
	Fair Value Measurement Using Significant Unobservable Inputs (Level 3)
	Three months ended March 31,
	2015			2014
	Derivatives (a)
	(In millions)
Beginning balance	$	20		$	(1	)
Total (losses)/gains included in earnings — realized/unrealized	(1		)	2
Purchases	(3		)	—
Transfers into Level 3 (b)	—			(1		)
Ending balance	$	16		$	—
Losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of March 31	$	(1	)	$	—
(a) Consists of derivative assets and liabilities, net.
(b) Transfers in/out of level 3 are related to the availability of external broker quotes and are valued as of the end of the reporting period.
GenOn Mid-Atlantic
The following tables present assets and liabilities (including affiliate amounts) measured and recorded at fair value on GenOn Mid-Atlantic's consolidated balance sheet on a recurring basis and their level within the fair value hierarchy:
	As of March 31, 2015
	Fair Value
	Level 1 (a)			Level 2 (a)			Level 3		Total
	(In millions)
Derivative assets:
Commodity contracts	$	166		$	190		$	22	$	378
Derivative liabilities:
Commodity contracts	$	75		$	115		$	6	$	196
(a) There were no transfers during the three months ended March 31, 2015, between Levels 1 and 2.
	As of December 31, 2014
	Fair Value
	Level 1 (a)			Level 2 (a)			Level 3		Total
	(In millions)
Derivative assets:
Commodity contracts	$	145		$	211		$	26	$	382
Derivative liabilities:
Commodity contracts	$	71		$	73		$	6	$	150
(a) There were no transfers during the year ended December 31, 2014, between Levels 1 and 2.
The following table reconciles, for the three months ended March 31, 2015, and 2014, the beginning and ending balances for GenOn Mid-Atlantic's derivatives that are recognized at fair value in the consolidated financial statements at least annually using significant unobservable inputs:
	Fair Value Measurement Using Significant Unobservable Inputs (Level 3)
	Three months ended March 31,
	2015			2014
	Derivatives (a)
	(In millions)
Beginning balance	$	20		$	—
Total losses included in earnings — realized/unrealized	(1		)	—
Purchases	(3		)	—
Ending balance	$	16		$	—
Losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of March 31	$	(1	)	$	—
(a) Consists of derivative assets and liabilities, net.
Derivative Fair Value Measurements
A portion of the Registrants' contracts are exchange-traded contracts with readily available quoted market prices. A majority of the Registrants' contracts are non-exchange-traded contracts valued using prices provided by external sources, primarily price quotations available through brokers or over-the-counter and on-line exchanges. The remainder of the assets and liabilities represent contracts for which external sources or observable market quotes are not available for the whole term or for certain delivery months. These contracts are valued using various valuation techniques including but not limited to internal models that apply fundamental analysis of the market and corroboration with similar markets. As of March 31, 2015, contracts valued with prices provided by models and other valuation techniques make up 4% of GenOn's derivative assets and 3% of GenOn's derivative liabilities, 4% of GenOn Americas Generation’s derivative assets and 3% of GenOn Americas Generation's derivative liabilities and 6% of GenOn Mid-Atlantic’s derivative assets and 3% of GenOn Mid-Atlantic's derivative liabilities.
The fair value of each contract is discounted using a risk free interest rate. In addition, the Registrants apply a credit reserve to reflect credit risk which is calculated based on published default probabilities. The Registrants' credit reserves were as follows:
	As of March 31, 2015			As of December 31, 2014
	(In millions)
GenOn	$	—		$	—
GenOn Americas Generation	—			—
GenOn Mid-Atlantic	2			2
Concentration of Credit Risk
In addition to the credit risk discussion as disclosed in Note 2, Summary of Significant Accounting Policies, to the Registrants' 2014 Form 10-K, the following is a discussion of the concentration of credit risk for the Registrants’ financial instruments. Credit risk relates to the risk of loss resulting from non-performance or non-payment by counterparties pursuant to the terms of their contractual obligations. The Registrants are exposed to counterparty credit risk through various activities including wholesale sales and fuel purchases.
Counterparty Credit Risk
The Registrants' counterparty credit risk policies are disclosed in their 2014 Form 10-K. As of March 31, 2015, GenOn's counterparty credit exposure was $361 million and GenOn held $4 million of collateral (cash and letters of credit) against those positions, resulting in a net exposure of $361 million. Approximately 83% of GenOn's exposure before collateral is expected to roll off by the end of 2016. As of March 31, 2015, GenOn Americas Generation’s counterparty credit exposure was $347 million and GenOn Americas Generation held $4 million of collateral (cash and letters of credit) against those positions, resulting in a net exposure of $347 million. Approximately 85% of GenOn Americas Generation’s exposure before collateral is expected to roll off by the end of 2016. As of March 31, 2015, GenOn Mid-Atlantic’s counterparty credit exposure was $83 million and GenOn Mid-Atlantic held no collateral (cash and letters or credit) against those positions, resulting in a net exposure of $83 million. 100% of GenOn Mid-Atlantic’s exposure before collateral is expected to roll off by the end of 2016. The following tables highlight net counterparty credit exposure by industry sector and by counterparty credit quality. Net counterparty credit exposure is defined as the aggregate net asset position for the Registrants with counterparties where netting is permitted under the enabling agreement and includes all cash flow, mark-to-market, NPNS and non-derivative transactions. The exposure is shown net of collateral held and includes amounts net of receivables or payables.
GenOn
Category	Net Exposure (a)
	(% of Total)
Financial institutions	72	%
Utilities, energy merchants, marketers and other	12
ISOs	16
Total as of March 31, 2015	100	%
Category	Net Exposure (a)
	(% of Total)
Investment grade	97	%
Below investment grade	2
Non-rated	1
Total as of March 31, 2015	100	%
(a) Counterparty credit exposure excludes transportation contracts because of the unavailability of market prices.
GenOn has counterparty credit risk exposure to certain counterparties, each of which represent more than 10% of the total net exposure discussed above. The aggregate of such counterparties' exposure was $285 million. Changes in hedge positions and market prices will affect credit exposure and counterparty concentration. Given the credit quality, diversification and term of the exposure in the portfolio, GenOn does not anticipate a material impact on its financial position or results of operations from nonperformance by any of its counterparties.
GenOn Americas Generation
Category	Net Exposure (a)
	(% of Total)
Financial institutions	75	%
Utilities, energy merchants, marketers and other	8
ISOs	17
Total as of March 31, 2015	100	%
Category	Net Exposure (a)
	(% of Total)
Investment grade	99	%
Non-rated	1
Total as of March 31, 2015	100	%
(a) Counterparty credit exposure excludes transportation contracts because of the unavailability of market prices.
GenOn Americas Generation has counterparty credit risk exposure to certain counterparties, each of which represent more than 10% of the total net exposure discussed above. The aggregate of such counterparties' exposure was $285 million. Changes in hedge positions and market prices will affect credit exposure and counterparty concentration. Given the credit quality, diversification and term of the exposure in the portfolio, GenOn Americas Generation does not anticipate a material impact on its financial position or results of operations from nonperformance by any of its counterparties.
GenOn Mid-Atlantic
Category	Net Exposure (a)
	(% of Total)
Financial institutions	100	%
Category	Net Exposure (a)
	(% of Total)
Investment grade	100	%
(a) Counterparty credit exposure excludes transportation contracts because of the unavailability of market prices.
GenOn Mid-Atlantic has counterparty credit risk exposure to certain counterparties, each of which represent more than 10% of the total net exposure discussed above. The aggregate of such counterparties' exposure was $83 million. Changes in hedge positions and market prices will affect credit exposure and counterparty concentration. Given the credit quality, diversification and term of the exposure in the portfolio, GenOn Mid-Atlantic does not anticipate a material impact on its financial position or results of operations from nonperformance by any of its counterparties.

Accounting_for_Derivative_Inst

Accounting for Derivative Instruments and Hedging Activities	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Accounting for Derivative Instruments and Hedging Activities (GenOn, GenOn Americas Generation, GenOn Mid-Atlantic)	Accounting for Derivative Instruments and Hedging Activities (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	This footnote should be read in conjunction with the complete description under Note 5, Accounting for Derivative Instruments and Hedging Activities, to the 2014 Form 10-K.
	Energy-Related Commodities (GenOn)
	As of March 31, 2015, GenOn had energy-related derivative financial instruments extending through 2017.
	Volumetric Underlying Derivative Transactions (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	The following table summarizes the net notional volume buy/(sell) of the Registrants’ open derivative transactions broken out by commodity, excluding those derivatives that qualified for the NPNS exception as of March 31, 2015, and December 31, 2014. Option contracts are reflected using delta volume. Delta volume equals the notional volume of an option adjusted for the probability that the option will be in-the-money at its expiration date.
			GenOn					GenOn Americas Generation					GenOn Mid-Atlantic
			Total Volume					Total Volume					Total Volume
			As of March 31, 2015			As of December 31, 2014		As of March 31, 2015			As of December 31, 2014		As of March 31, 2015			As of December 31, 2014
	Commodity	Units	(In millions)
	Coal	Short Ton	7			8		5			5		5			5
	Natural Gas	MMBtu	85			-21		(38	)		-74		(52	)		-79
	Power	MWh	(41	)		-36		(16	)		-16		(14	)		-15
	The decrease in the natural gas position was the result of buying natural gas to convert fixed price natural gas hedges into fixed price power hedges, as well as the settlement of positions during the period.
	Fair Value of Derivative Instruments (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	The following tables summarize the fair value within the derivative instrument valuation on the balance sheet:
	GenOn
		Fair Value
		Derivative Assets								Derivative Liabilities
		31-Mar-15				31-Dec-14				31-Mar-15				31-Dec-14
		(In millions)
	Derivatives Not Designated as Cash Flow Hedges:
	Commodity contracts current	$	498			$	602			$	431			$	417
	Commodity contracts long-term	246				205				139				72
	Total Derivatives Not Designated as Cash Flow Hedges	$	744			$	807			$	570			$	489
	GenOn Americas Generation
		Fair Value
		Derivative Assets								Derivative Liabilities
		31-Mar-15				31-Dec-14				31-Mar-15				31-Dec-14
		(In millions)
	Derivatives Not Designated as Cash Flow Hedges:
	Commodity contracts current	$	755			$	852			$	673			$	674
	Commodity contracts long-term	335				256				231				135
	Total Derivatives Not Designated as Cash Flow Hedges	$	1,090			$	1,108			$	904			$	809
	GenOn Mid-Atlantic
		Fair Value
		Derivative Assets								Derivative Liabilities
		31-Mar-15				31-Dec-14				31-Mar-15				31-Dec-14
		(In millions)
	Derivatives Not Designated as Cash Flow Hedges:
	Commodity contracts current	$	228			$	241			$	148			$	128
	Commodity contracts long-term	150				141				48				22
	Total Derivatives Not Designated as Cash Flow Hedges	$	378			$	382			$	196			$	150
	The Registrants have elected to present derivative assets and liabilities on the balance sheet on a trade-by-trade basis and do not offset amounts at the counterparty master agreement level. In addition, collateral received or paid on the Registrants' derivative assets or liabilities are recorded on a separate line item on the balance sheet. The following tables summarize the offsetting of derivatives by counterparty master agreement level and collateral received or paid:
	GenOn
			Gross Amounts Not Offset in the Statement of Financial Position
	Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
	March 31, 2015		(In millions)
	Commodity contracts:
	Derivative assets		$	718			$	(466	)		$	(25	)		$	227
	Derivative assets - affiliate		26				(26		)		—				—
	Derivative liabilities		(530		)		466				—				(64		)
	Derivative liabilities - affiliate		(40		)		26				14				—
	Total derivative instruments		$	174			$	—			$	(11	)		$	163
	GenOn Americas Generation
			Gross Amounts Not Offset in the Statement of Financial Position
	Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
	March 31, 2015		(In millions)
	Commodity contracts:
	Derivative assets		$	718			$	(466	)		$	(25	)		$	227
	Derivative assets - affiliate		372				(372		)		—				—
	Derivative liabilities		(530		)		466				—				(64		)
	Derivative liabilities - affiliate		(374		)		372				2				—
	Total derivative instruments		$	186			$	—			$	(23	)		$	163
GenOn Mid-Atlantic
		Gross Amounts Not Offset in the Statement of Financial Position
Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
March 31, 2015		(In millions)
Commodity contracts:
Derivative assets		$	83			$	(1	)		$	—			$	82
Derivative assets - affiliate		295				(195		)		—				100
Derivative liabilities		(1		)		1				—				—
Derivative liabilities - affiliate		(195		)		195				—				—
Total derivative instruments		$	182			$	—			$	—			$	182
GenOn
		Gross Amounts Not Offset in the Statement of Financial Position
Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
December 31, 2014		(In millions)
Commodity contracts:
Derivative assets		$	786			$	(425	)		$	(54	)		$	307
Derivative assets - affiliate		21				(21		)		—				—
Derivative liabilities		(451		)		425				—				(26		)
Derivative liabilities - affiliate		(38		)		21				17				—
Total derivative instruments		$	318			$	—			$	(37	)		$	281
GenOn Americas Generation
		Gross Amounts Not Offset in the Statement of Financial Position
Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
December 31, 2014		(In millions)
Commodity contracts:
Derivative assets		$	787			$	(425	)		$	(54	)		$	308
Derivative assets - affiliate		321				(321		)		—				—
Derivative liabilities		(451		)		425				—				(26		)
Derivative liabilities - affiliate		(358		)		321				17				(20		)
Total derivative instruments		$	299			$	—			$	(37	)		$	262
GenOn Mid-Atlantic
		Gross Amounts Not Offset in the Statement of Financial Position
Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
December 31, 2014		(In millions)
Commodity contracts:
Derivative assets		$	100			$	—			$	—			$	100
Derivative assets - affiliate		282				(149		)		—				133
Derivative liabilities		(1		)		—				—				(1		)
Derivative liabilities - affiliate		(149		)		149				—				—
Total derivative instruments		$	232			$	—			$	—			$	232
Impact of Derivative Instruments on the Statements of Operations (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
Unrealized gains and losses associated with changes in the fair value of derivative instruments not accounted for as cash flow hedges are reflected in current period earnings.
The following tables summarize the pre-tax effects of economic hedges that have not been designated as cash flow hedges and trading activity on the Registrants’ statements of operations. These amounts are included within operating revenues and cost of operations.
GenOn
	Three months ended March 31,
(In millions)	2015				2014
Unrealized mark-to-market results
Reversal of previously recognized unrealized gains on settled positions related to economic hedges	$	(86	)		$	(60	)
Net unrealized losses on open positions related to economic hedges	(62		)		(171		)
Total unrealized mark-to-market losses for economic hedging activities	(148		)		(231		)
Reversal of previously recognized unrealized gains on settled positions related to trading activity	—				(1		)
Total unrealized mark-to-market losses for trading activity	—				(1		)
Total unrealized losses	$	(148	)		$	(232	)
	Three months ended March 31,
(In millions)	2015				2014
Revenue from operations — energy commodities	$	(99	)		$	(225	)
Cost of operations	(49		)		(7		)
Total impact to statements of operations	$	(148	)		$	(232	)
GenOn Americas Generation
	Three months ended March 31,
(In millions)	2015				2014
Unrealized mark-to-market results
Reversal of previously recognized unrealized gains on settled positions related to economic hedges	$	(95	)		$	(57	)
Net unrealized losses on open positions related to economic hedges	(22		)		(93		)
Total unrealized mark-to-market losses for economic hedging activities	(117		)		(150		)
Reversal of previously recognized unrealized gains on settled positions related to trading activity	—				(1		)
Total unrealized mark-to-market losses for trading activity	—				(1		)
Total unrealized losses	$	(117	)		$	(151	)
	Three months ended March 31,
(In millions)	2015				2014
Revenue from operations — energy commodities	$	(86	)		$	(145	)
Cost of operations	(31		)		(6		)
Total impact to statements of operations	$	(117	)		$	(151	)
GenOn Mid-Atlantic
	Three months ended March 31,
(In millions)	2015				2014
Unrealized mark-to-market results
Reversal of previously recognized unrealized gains on settled positions related to economic hedges	$	(27	)		$	(63	)
Net unrealized losses on open positions related to economic hedges	(28		)		(92		)
Total unrealized losses	$	(55	)		$	(155	)
	Three months ended March 31,
(In millions)	2015				2014
Revenue from operations — energy commodities	$	(24	)		$	(150	)
Cost of operations	(31		)		(5		)
Total impact to statements of operations	$	(55	)		$	(155	)
Credit Risk Related Contingent Features (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
Certain of GenOn and GenOn Americas Generation’s hedging agreements contain provisions that require the Registrants to post additional collateral if the counterparty determines that there has been deterioration in credit quality, generally termed "adequate assurance" under the agreements, or require the Registrants to post additional collateral if there were a one notch downgrade in the Registrants’ credit rating. The collateral required for contracts that have adequate assurance clauses that are in net liability positions as of March 31, 2015, was $39 million for GenOn and GenOn Americas Generation. The collateral required for contracts with credit rating contingent features that are in a net liability position as of March 31, 2015, was $1 million for GenOn and GenOn Americas Generation. In addition, GenOn and GenOn Americas Generation are parties to certain marginable agreements under which they have net liability positions, but the counterparties have not called for collateral due, which was zero for GenOn and GenOn Americas Generation as of March 31, 2015. As of March 31, 2015, GenOn Mid-Atlantic is not party to certain marginable agreements under which they have net liability positions, but the counterparties have not called for collateral due.
See Note 3, Fair Value of Financial Instruments, for discussion regarding concentration of credit risk.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes (GenOn, GenOn Americas Generation, GenOn Mid-Atlantic)	Income Taxes (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	GenOn
	GenOn’s income tax expense/(benefit) consisted of the following:
		Three months ended March 31,
	(In millions except otherwise noted)	2015			2014
	(Loss)/Income before income taxes	$	(12	)	$	4
	Income tax (benefit)/expense	(1		)	1
	Effective tax rate	8.3		%	25		%
	GenOn
	For the three months ended March 31, 2015, and 2014, respectively, GenOn's overall effective tax rate was lower than the statutory rate of 35% primarily due to a change in the valuation allowance, partially offset by the impact of state income taxes.
	GenOn Americas Generation
	GenOn Americas Generation's allocated income taxes resulting from its operations for the three months ended March 31, 2015, and 2014 were $0. GenOn Americas Generation's pro forma income taxes resulting from its operations for the three months ended March 31, 2015, and 2014 are $0 due to the valuation allowance recorded on its stand-alone financial results.
	GenOn Mid-Atlantic
	GenOn Mid-Atlantic’s allocated income taxes resulting from its operations are $0 for the three months ended March 31, 2015, and 2014. The pro forma income tax provision attributable to income before taxes is a tax expense of $8 million and $17 million during the three months ended March 31, 2015, and 2014, respectively. The balance of GenOn Mid-Atlantic's pro forma deferred income taxes is a net deferred tax asset of $43 million and $51 million as of March 31, 2015, and December 31, 2014, respectively.

Related_Party_Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Related Party Transactions (GenOn, GenOn Americas Generaiton, GenOn Mid-Atlantic)	Related Party Transactions (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	Services Agreement with NRG
	NRG provides GenOn with various management, personnel and other services, which include human resources, regulatory and public affairs, accounting, tax, legal, information systems, treasury, risk management, commercial operations, and asset management, as set forth in its services agreement with NRG, or the Services Agreement. The initial term of the Services Agreement was through December 31, 2013, with an automatic renewal absent a request for termination. The fee charged is determined based on a fixed amount as described in the Services Agreement and was calculated based on historical GenOn expenses prior to the NRG Merger. The annual fees under the Services Agreement are approximately $193 million. NRG charges these fees on a monthly basis, less amounts incurred directly by GenOn. Management has concluded that this method of charging overhead costs is reasonable. For the three months ended March 31, 2015 and March 31, 2014, GenOn recorded costs related to these services of $47 million and $33 million, respectively, as selling, general and administrative — affiliate.
	Under the Services Agreement, NRG also provides GenOn Americas Generation and GenOn Mid-Atlantic with various management, personnel and other services consistent with those set forth in the Services Agreement discussed above between NRG and GenOn. GenOn's costs incurred under the Services Agreement with NRG are allocated to its subsidiaries based on each operating subsidiary's planned operating expenses relative to all operating subsidiaries of GenOn. These allocations and charges are not necessarily indicative of what would have been incurred had GenOn Americas Generation and GenOn Mid-Atlantic been unaffiliated entities. Management has concluded that this method of charging overhead costs is reasonable.
	The following costs were incurred under these arrangements:
	GenOn Americas Generation
		Three months ended March 31,
		2015		2014
		(In millions)
	Allocated costs:
	Cost of operations — affiliate	$	—	$	2
	Selling, general and administrative — affiliate	21		18
	Total	$	21	$	20
	GenOn Mid-Atlantic
		Three months ended March 31,
		2015		2014
		(In millions)
	Allocated costs:
	Cost of operations — affiliate	$	—	$	1
	Selling, general and administrative — affiliate	15		15
	Total	$	15	$	16
	Credit Agreement with NRG (GenOn)
	In connection with the closing of the NRG Merger, GenOn and GenOn Americas entered into a secured intercompany revolving credit agreement with NRG.  This credit agreement provides for a $500 million revolving credit facility, all of which is available for revolving loans and letters of credit. At March 31, 2015, and December 31, 2014, $240 million and $237 million, respectively, of letters of credit were issued and outstanding under the NRG credit agreement, of which $163 million and $173 million, respectively, were issued on behalf of GenOn Americas Generation. At March 31, 2015, and December 31, 2014, no loans were outstanding under this credit agreement.  In connection with the execution of the agreement, certain of GenOn's subsidiaries, as guarantors, entered into a guarantee agreement pursuant to which these guarantors guaranteed amounts borrowed and obligations incurred under the credit agreement. The credit agreement has a three year maturity and is payable at maturity, subject to certain exceptions primarily related to asset sales not in the ordinary course of business and borrowings of debt. In addition, the guarantors are restricted from incurring additional liens on their assets. At GenOn's election, the interest rate per year applicable to the loans under the credit agreement will be determined by reference to either (i) the base rate plus 2.50% per year or (ii) the LIBOR rate plus 3.50% per year. In addition, the credit agreement contains customary covenants and events of default.
	Intercompany Cash Management Program (GenOn Americas Generation)
	GenOn Americas Generation and certain of its subsidiaries participate in separate intercompany cash management programs whereby cash balances at GenOn Americas Generation and the respective participating subsidiaries are transferred to central concentration accounts to fund working capital and other needs of the respective participants. The balances under this program are reflected as notes receivable — affiliate and accounts receivable — affiliate or notes payable — affiliate and accounts payable — affiliate, as appropriate. The balances are due on demand and accrue interest on the net position, which is payable quarterly, at a rate determined by GenOn Energy Holdings, a wholly owned subsidiary of GenOn. At March 31, 2015, and December 31, 2014, GenOn Americas Generation had a net current note receivable — affiliate from GenOn Energy Holdings of $331 million related to its historical intercompany cash management activity. For the three months ended March 31, 2015, and 2014, GenOn Americas Generation earned an insignificant amount of net interest income related to these notes. Additionally, at March 31, 2015, and December 31, 2014, GenOn Americas Generation had an accounts receivable — affiliate of $165 million and $118 million, respectively, with GenOn Energy Holdings.

Commitments_and_Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies (GenOn, GenOn Americas Generation, GenOn Mid-Atlantic)	Commitments and Contingencies (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	This footnote should be read in conjunction with the complete description under Note 16, Commitments and Contingencies, to the Registrants' 2014 Form 10-K.
	Contingencies
	The Registrants’ material legal proceedings are described below. The Registrants believe that they have valid defenses to these legal proceedings and intend to defend them vigorously. The Registrants record reserves for estimated losses from contingencies when information available indicates that a loss is probable and the amount of the loss, or range of loss, can be reasonably estimated. In addition, legal costs are expensed as incurred. Management has assessed each of the following matters based on current information and made a judgment concerning its potential outcome, considering the nature of the claim, the amount and nature of damages sought, and the probability of success. Unless specified below, the Registrants are unable to predict the outcome of these legal proceedings or reasonably estimate the scope or amount of any associated costs and potential liabilities. As additional information becomes available, management adjusts its assessment and estimates of such contingencies accordingly. Because litigation is subject to inherent uncertainties and unfavorable rulings or developments, it is possible that the ultimate resolution of the Registrants’ liabilities and contingencies could be at amounts that are different from currently recorded reserves and that such difference could be material.
	In addition to the legal proceedings noted below, the Registrants are parties to other litigation or legal proceedings arising in the ordinary course of business. In management's opinion, the disposition of these ordinary course matters will not materially adversely affect the Registrants’ respective consolidated financial position, results of operations, or cash flows.
	Actions Pursued by MC Asset Recovery (GenOn) — With Mirant Corporation's emergence from bankruptcy protection in 2006, certain actions filed by GenOn Energy Holdings and some of its subsidiaries against third parties were transferred to MC Asset Recovery, a wholly owned subsidiary of GenOn Energy Holdings.  MC Asset Recovery is governed by a manager who is independent of NRG and GenOn.  MC Asset Recovery is a disregarded entity for income tax purposes. Under the remaining action transferred to MC Asset Recovery, MC Asset Recovery seeks to recover damages from Commerzbank AG and various other banks, or the Commerzbank Defendants, for alleged fraudulent transfers that occurred prior to Mirant's bankruptcy proceedings.  In December 2010, the U.S. District Court for the Northern District of Texas dismissed MC Asset Recovery's complaint against the Commerzbank Defendants.  In January 2011, MC Asset Recovery appealed the District Court's dismissal of its complaint against the Commerzbank Defendants to the U.S. Court of Appeals for the Fifth Circuit.  In March 2012, the Court of Appeals reversed the District Court's dismissal and reinstated MC Asset Recovery's amended complaint against the Commerzbank Defendants.  If MC Asset Recovery succeeds in obtaining any recoveries from the Commerzbank Defendants, the Commerzbank Defendants have asserted that they will seek to file claims in Mirant's bankruptcy proceedings for the amount of those recoveries.  GenOn Energy Holdings would vigorously contest the allowance of any such claims. If the Commerzbank Defendants were to receive an allowed claim as a result of a recovery by MC Asset Recovery on its claims against them, GenOn Energy Holdings would retain from the net amount recovered by MC Asset Recovery an amount equal to the dollar amount of the resulting allowed claim.
	Pending Natural Gas Litigation (GenOn) — GenOn is party to several lawsuits, certain of which are class action lawsuits, in state and federal courts in Kansas, Missouri, Nevada and Wisconsin. These lawsuits were filed in the aftermath of the California energy crisis in 2000 and 2001 and the resulting FERC investigations and relate to alleged conduct to increase natural gas prices in violation of antitrust and similar laws. The lawsuits seek treble or punitive damages, restitution and/or expenses. The lawsuits also name as parties a number of energy companies unaffiliated with NRG. In July 2011, the U.S. District Court for the District of Nevada, which was handling four of the five cases, granted the defendants' motion for summary judgment and dismissed all claims against GenOn in those cases. The plaintiffs appealed to the U.S. Court of Appeals for the Ninth Circuit which reversed the decision of the District Court. On August 26, 2013, GenOn along with the other defendants in the lawsuit filed a petition for a writ of certiorari to the U.S. Supreme Court challenging the Court of Appeals' decision. On July 1, 2014, the U.S. Supreme Court granted the petition. On April 21, 2015, the U.S. Supreme Court affirmed the Ninth Circuit’s holding that plaintiffs’ state antitrust law claims are not field-preempted.  The U.S. Supreme Court left open whether the claims were preempted on the basis of conflict preemption and directed that the case be sent to the U.S. District Court for the District of Nevada for further proceedings.
	In September 2012, the State of Nevada Supreme Court, which was handling the remaining case, affirmed dismissal by the Eighth Judicial District Court for Clark County, Nevada of all plaintiffs' claims against GenOn. In February 2013, the plaintiffs in the Nevada case filed a petition for a writ of certiorari to the U.S. Supreme Court. In June 2013, the U.S. Supreme Court denied the petition for a writ of certiorari, thereby ending one of the five lawsuits. GenOn has agreed to indemnify CenterPoint against certain losses relating to these lawsuits.
	Cheswick Class Action Complaint (GenOn) — In April 2012, a putative class action lawsuit was filed against GenOn in the Court of Common Pleas of Allegheny County, Pennsylvania alleging that emissions from the Cheswick generating facility have damaged the property of neighboring residents.  GenOn disputes these allegations.  Plaintiffs have brought nuisance, negligence, trespass and strict liability claims seeking both damages and injunctive relief.  Plaintiffs seek to certify a class that consists of people who own property or live within one mile of the plant. In July 2012, GenOn removed the lawsuit to the U.S. District Court for the Western District of Pennsylvania. In October 2012, the District Court granted GenOn’s motion to dismiss, which plaintiffs appealed to the U.S. Court of Appeals for the Third Circuit. On August 20, 2013, the Court of Appeals reversed the decision of the District Court. On September 3, 2013, GenOn filed a petition for rehearing with the Court of Appeals which was subsequently denied. In February 2014, NRG filed a petition for a writ of certiorari to the U.S. Supreme Court seeking review and reversal of the Court of Appeals' decision. On June 2, 2014, the U.S. Supreme Court denied the petition for a writ of certiorari. Following the U.S. Supreme Court's denial of GenOn's petition for a writ of certiorari, the case continued to be litigated before the U.S. District Court for the Western District of Pennsylvania. After briefing by the parties on GenOn's motion to strike class allegations in the complaint, the court granted GenOn's motion, but allowed the plaintiffs the opportunity to re-file their complaint. On February 3, 2015, plaintiffs sought leave to file an amended complaint, which NRG is contesting.
	Maryland Department of the Environment v. GenOn Chalk Point and GenOn Mid-Atlantic — On January 25, 2013, Food & Water Watch, the Patuxent Riverkeeper and the Potomac Riverkeeper (together, the Citizens Group) sent GenOn Mid-Atlantic a letter alleging that the Chalk Point, Dickerson and Morgantown generating facilities were violating the terms of the three National Pollution Discharge Elimination System permits by discharging nitrogen and phosphorous in excess of the limits in each permit. On March 21, 2013, the MDE sent GenOn Mid-Atlantic a similar letter with respect to the Chalk Point and Dickerson generating facilities, threatening to sue within 60 days if the generating facilities were not brought into compliance. On June 11, 2013, the Maryland Attorney General on behalf of the MDE filed a complaint in the U.S. District Court for the District of Maryland alleging violations of the CWA and Maryland environmental laws related to water. The lawsuit is ongoing and seeks injunctive relief and civil penalties in excess of $100,000. The Registrants do not expect the resolution of this matter to have a material impact on the Registrants' consolidated financial position, results of operations, or cash flows.
	Chapter 11 Proceedings (GenOn and GenOn Americas Generation) — In July 2003, and various dates thereafter, the Mirant Debtors filed voluntary petitions in the Bankruptcy Court for relief under Chapter 11 of the U.S. Bankruptcy Code. GenOn Energy Holdings and most of the other Mirant Debtors emerged from bankruptcy on January 3, 2006, when the Plan became effective. The remaining Mirant Debtors emerged from bankruptcy on various dates in 2007. Approximately 461,000 of the shares of GenOn Energy Holdings common stock to be distributed under the Plan have not yet been distributed and have been reserved for distribution with respect to claims disputed by the Mirant Debtors that have not been resolved. Upon the Mirant/RRI Merger, those reserved shares converted into a reserve for approximately 1.3 million shares of GenOn common stock. Upon the NRG Merger, those reserved shares converted into a reserve for approximately 159,000 shares of NRG common stock. Under the terms of the Plan, upon the resolution of such a disputed claim, the claimant will receive the same pro rata distributions of common stock, cash, or both as previously allowed claims, regardless of the price at which the common stock is trading at the time the claim is resolved. If the aggregate amount of any such payouts results in the number of reserved shares being insufficient, additional shares of common stock may be issued to address the shortfall.

Regulatory_Matters

Regulatory Matters	3 Months Ended
	Mar. 31, 2015
Regulatory Matters Disclosure [Abstract]
Regulatory Matters (GenOn, GenOn Americas Generation, GenOn Mid-Atlantic)	Regulatory Matters (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	This footnote should be read in conjunction with the complete description under Note 17, Regulatory Matters, to the Registrants' 2014 Form 10-K.
	The Registrants operate in a highly regulated industry and are subject to regulation by various federal and state agencies. As such, the Registrants are affected by regulatory developments at both the federal and state levels and in the regions in which they operate. In addition, the Registrants are subject to the market rules, procedures, and protocols of the various ISO and RTO markets in which they participate. These power markets are subject to ongoing legislative and regulatory changes that may impact the Registrants' wholesale business.
	In addition to the regulatory proceedings noted below, the Registrants are parties to other regulatory proceedings arising in the ordinary course of business or have other regulatory exposure. In management's opinion, the disposition of these ordinary course matters will not materially adversely affect the Registrants’ respective consolidated financial position, results of operations, or cash flows.
	National
	Court Rejects FERC's Jurisdiction Over Demand Response — On May 23, 2014, the U.S. Court of Appeals for the District of Columbia Circuit vacated FERC’s rules (known as Order No. 745) that allowed demand response resources to participate in FERC-jurisdictional energy markets. The Court of Appeals held that the Federal Power Act does not authorize FERC to exercise jurisdiction over demand response and that instead demand response is part of the retail market over which the states have jurisdiction. The specific order being challenged related to energy market compensation, but this ruling also calls into question whether demand response will be permitted to participate in the capacity markets in the future. Parties including the U.S. Solicitor General have filed petitions for a writ of certiorari with the U.S. Supreme Court. On May 4, 2015, the U.S. Supreme Court granted certiorari on two questions: first, on whether the FPA gives FERC jurisdiction over demand response, and second, whether FERC’s Order No. 745 correctly determined the level of compensation paid to generators participating in the energy markets. The eventual outcome of this proceeding could result in refunds of payments made for non-jurisdictional services and resettlement of wholesale markets, but it is not possible to predict the outcome or estimate the impact on the Registrants at this time.
	East Region (GenOn)
	Montgomery County Station Power Tax — On December 20, 2013, NRG received a letter from Montgomery County, Maryland requesting payment of an energy tax for the consumption of station power at the Dickerson Facility over the previous three years. Montgomery County seeks payment in the amount of $22 million, which includes tax, interest and penalties. NRG is disputing the applicability of the tax. On December 17, 2014, the Maryland Tax Court heard oral arguments from the parties. Subsequently, post hearing briefs were filed. The decision is pending.

Environmental_Matters

Environmental Matters	3 Months Ended
	Mar. 31, 2015
Environmental Matters Disclosure [Abstract]
Environmental Matters (GenOn, GenOn Americas Generation, GenOn Mid-Atlantic)	Environmental Matters (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic)
	This footnote should be read in conjunction with the complete description under Note 18, Environmental Matters, to the Registrants' 2014 Form 10-K.
	The Registrants are subject to a wide range of environmental laws in the development, construction, ownership and operation of projects. These laws generally require that governmental permits and approvals be obtained before construction and during operation of power plants. The Registrants are also subject to laws and regulations surrounding the protection of wildlife, including migratory birds, eagles and threatened and endangered species. Environmental laws have become increasingly stringent and the Registrants expect this trend to continue. The electric generation industry is likely to face new requirements to address various emissions, including GHG, as well as combustion byproducts, water discharge and use, and threatened and endangered species. In general, future laws are expected to require the addition of emissions controls or other environmental controls or to impose certain restrictions on the operations of the Registrants' facilities, which could have a material effect on the Registrants' operations.
	The EPA finalized CSAPR in 2011, which was intended to replace CAIR in January 2012. In December 2011, the U.S. Court of Appeals for the District of Columbia Circuit stayed the implementation of CSAPR and then issued an opinion in August 2012 vacating CSAPR and keeping CAIR in place until the EPA could replace it. On April 29, 2014, the U.S. Supreme Court reversed and remanded the D.C. Circuit's decision. In October 2014, the D.C. Circuit lifted the stay of CSAPR. In response, the EPA issued an interim final rule in November 2014 to amend the CSAPR compliance dates. Accordingly, CSAPR replaced CAIR on January 1, 2015. On February 25, 2015, the D.C. Circuit held oral argument regarding several unresolved legal issues and the Registrants expect a decision in the second quarter of 2015. While the Registrants cannot predict the final outcome of the ongoing litigation, the Registrants believe their investment in pollution controls and cleaner technologies coupled with planned plant retirements should leave the fleet well positioned for compliance.
	In December 2014, the EPA proposed making the NAAQS for ozone more stringent. The EPA anticipates promulgating a more stringent ozone NAAQS by October 2015. A more stringent NAAQS would obligate the states to develop plans to reduce NOx (an ozone precursor), which might affect some of the Registrants' units.
	In February 2012, the EPA promulgated standards to control emissions of HAPs from coal and oil-fired electric generating units. The rule established limits for mercury, non-mercury metals, certain organics and acid gases, which limits must be met beginning in April 2015 (with some units getting a 1-year extension). In November 2014, the U.S. Supreme Court agreed to review the D.C. Circuit decision that denied the petitions seeking to vacate MATS but the review is limited to whether the EPA unreasonably refused to consider costs in determining whether it is appropriate to regulate hazardous air pollutants emitted by electric generating units. The oral argument in the U.S. Supreme Court occurred in March 2015, and the Registrants expect a decision in the second quarter of 2015.
	In January 2014, the EPA re-proposed the NSPS for CO2 emissions from new fossil-fuel-fired electric generating units that had been previously proposed in April 2012. The re-proposed standards are 1,000 pounds of CO2 per MWh for large gas units and 1,100 pounds of CO2 per MWh for coal units and small gas units. Proposed standards are in effect until a final rule is published or another rule is re-proposed. In June 2014, the EPA proposed a rule that would require states to develop CO2 standards that would apply to existing fossil-fueled generating facilities. Specifically, the EPA proposed state-specific rate-based goals for CO2 emissions, as well as guidelines for states to follow in developing plans to achieve the state-specific goals. The EPA anticipates finalizing both of these rules in the summer of 2015.
	Water
	In August 2014, the EPA finalized the regulation regarding once through cooling from existing facilities to address impingement and entrainment concerns. The Registrants anticipate that more stringent requirements will be incorporated into some of their water discharge permits over the next several years.
	Byproducts, Wastes, Hazardous Materials and Contamination
	In April 2015, the EPA finalized the rule regulating byproducts of coal combustion (e.g., ash and gypsum) as solid wastes under the RCRA. In 2010, the EPA had proposed two alternatives. Under the first proposal, these byproducts would be regulated as solid wastes. Under the second proposal, these byproducts would have been regulated as “special wastes” in a manner similar to the regulation of hazardous waste with an exception for certain types of beneficial use of these byproducts. The second alternative would have imposed significantly more stringent requirements and materially increased the cost of disposal of coal combustion byproducts. The Registrants are evaluating the impact of the new rule on their results of operations, financial condition and cash flows.
	East Region
	Maryland Environmental Regulations — In December 2014, MDE proposed in the Maryland Register a regulation regarding NOx emissions from coal-fired electric generating units, which if finalized would have required by 2020 the Registrants (at each of the three Dickerson coal-fired units and the Chalk Point coal-fired unit that does not have an SCR) to (1) install and operate an SCR; (2) retire the unit; or (3) convert the fuel source from coal to natural gas. Earlier this year, a new administration decided not to finalize the regulation as proposed. Later this year, the Registrants expect MDE to propose revised regulations to address future NOx reductions, which when finalized may negatively affect certain of the Registrants' coal-fired units in Maryland.
	Environmental Capital Expenditures
	Based on current rules, technology and plans based on proposed rules, GenOn estimates that environmental capital expenditures from 2015 through 2019 required to meet GenOn's regulatory environmental laws will be approximately $57 million for GenOn, which includes $19 million for GenOn Americas Generation. The amount for GenOn Americas Generation includes $13 million for GenOn Mid-Atlantic.

Basis_of_Presentation_Reclassi

Basis of Presentation Reclassification (Policies)	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Reclassification, Policy [Policy Text Block]	Reclassifications
	Certain prior year amounts have been reclassified for comparative purposes.  The reclassification did not affect results from operations.

Basis_of_Presentation_Use_of_E

Basis of Presentation Use of Estimates (Policies)	3 Months Ended
	Mar. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Use of Estimates, Policy [Policy Text Block]	Use of Estimates
	The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities at the date of the financial statements, disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from these estimates.

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
Fair Value of Financial Instruments
Estimated carrying amounts and fair values of recorded financial instruments not carried at fair market value	The estimated carrying amounts and fair values of GenOn and GenOn Americas Generation’s debt are as follows:
	GenOn
		As of March 31, 2015						As of December 31, 2014
		Carrying Amount			Fair Value			Carrying Amount		Fair Value
		(In millions)
	Long and short-term debt	$	3,107		$	2,829		$	3,122	$	2,706
Assets and liabilities measured and recorded at fair value on the consolidated balance sheets on a recurring basis	GenOn
	The following tables present assets and liabilities (including affiliate amounts) measured and recorded at fair value on GenOn’s consolidated balance sheet on a recurring basis and their level within the fair value hierarchy:
		As of March 31, 2015
		Fair Value
		Level 1 (a)			Level 2 (a)			Level 3		Total
		(In millions)
	Derivative assets:
	Commodity contracts	$	197		$	516		$	31	$	744
	Derivative liabilities:
	Commodity contracts	$	118		$	437		$	15	$	570
	Other assets (b)	$	20		$	—		$	—	$	20
	(a) There were no transfers during the three months ended March 31, 2015, between Levels 1 and 2.
	(b) Relates to mutual funds held in a rabbi trust for non-qualified deferred compensation plans for certain key and highly compensated employees.
		As of December 31, 2014
		Fair Value
		Level 1 (a)			Level 2 (a)			Level 3		Total
		(In millions)
	Derivative assets:
	Commodity contracts	$	179		$	582		$	46	$	807
	Derivative liabilities:
	Commodity contracts	$	105		$	371		$	13	$	489
	Other assets (b)	$	21		$	—		$	—	$	21
	(a) There were no transfers during the year ended December 31, 2014, between Levels 1 and 2.
	(b) Relates to mutual funds held in a rabbi trust for non-qualified deferred compensation plans for certain key and highly compensated employees.
Reconciliation of beginning and ending balances for financial instruments that are recognized at fair value in the consolidated financial statements at least annually using significant unobservable inputs	The following table reconciles, for the three months ended March 31, 2015, and 2014, the beginning and ending balances for derivatives that are recognized at fair value in GenOn's consolidated financial statements at least annually using significant unobservable inputs:
		Fair Value Measurement Using Significant Unobservable Inputs (Level 3)
		Three months ended March 31,
		2015			2014
		Derivatives (a)
		(In millions)
	Beginning balance	$	33		$	(4	)
	Total (losses)/gains included in earnings — realized/unrealized	(13		)	4
	Purchases	(4		)	—
	Transfers into Level 3 (b)	—			(2		)
	Ending balance	$	16		$	(2	)
	Losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of March 31	$	(7	)	$	—
	(a) Consists of derivative assets and liabilities, net.
	(b) Transfers in/out of level 3 are related to the availability of external broker quotes and are valued as of the end of the reporting period.
Schedule of credit reserves for derivative contract assets	The Registrants' credit reserves were as follows:
		As of March 31, 2015			As of December 31, 2014
		(In millions)
	GenOn	$	—		$	—
	GenOn Americas Generation	—			—
	GenOn Mid-Atlantic	2			2
Net counterparty credit exposure by industry sector and by counterparty credit quality	The following tables highlight net counterparty credit exposure by industry sector and by counterparty credit quality. Net counterparty credit exposure is defined as the aggregate net asset position for the Registrants with counterparties where netting is permitted under the enabling agreement and includes all cash flow, mark-to-market, NPNS and non-derivative transactions. The exposure is shown net of collateral held and includes amounts net of receivables or payables.
	GenOn
	Category	Net Exposure (a)
		(% of Total)
	Financial institutions	72	%
	Utilities, energy merchants, marketers and other	12
	ISOs	16
	Total as of March 31, 2015	100	%
	Category	Net Exposure (a)
		(% of Total)
	Investment grade	97	%
	Below investment grade	2
	Non-rated	1
	Total as of March 31, 2015	100	%
	(a) Counterparty credit exposure excludes transportation contracts because of the unavailability of market prices.
GenOn Americas Generation
Fair Value of Financial Instruments
Estimated carrying amounts and fair values of recorded financial instruments not carried at fair market value	GenOn Americas Generation
		As of March 31, 2015						As of December 31, 2014
		Carrying Amount			Fair Value			Carrying Amount		Fair Value
		(In millions)
	Long and short-term debt	$	926		$	796		$	929	$	720
Assets and liabilities measured and recorded at fair value on the consolidated balance sheets on a recurring basis	GenOn Americas Generation
	The following tables present assets and liabilities (including affiliate amounts) measured and recorded at fair value on GenOn Americas Generation's consolidated balance sheet on a recurring basis and their level within the fair value hierarchy:
		As of March 31, 2015
		Fair Value
		Level 1 (a)			Level 2 (a)			Level 3		Total
		(In millions)
	Derivative assets:
	Commodity contracts	$	235		$	816		$	39	$	1,090
	Derivative liabilities:
	Commodity contracts	$	146		$	735		$	23	$	904
	(a) There were no transfers during the three months ended March 31, 2015, between Levels 1 and 2.
		As of December 31, 2014
		Fair Value
		Level 1 (a)			Level 2 (a)			Level 3		Total
		(In millions)
	Derivative assets:
	Commodity contracts	$	208		$	848		$	52	$	1,108
	Derivative liabilities:
	Commodity contracts	$	137		$	640		$	32	$	809
	(a) There were no transfers during the year ended December 31, 2014, between Levels 1 and 2.
Reconciliation of beginning and ending balances for financial instruments that are recognized at fair value in the consolidated financial statements at least annually using significant unobservable inputs	The following table reconciles, for the three months ended March 31, 2015, and 2014, the beginning and ending balances for GenOn Americas Generation's derivatives that are recognized at fair value in the consolidated financial statements at least annually using significant unobservable inputs:
		Fair Value Measurement Using Significant Unobservable Inputs (Level 3)
		Three months ended March 31,
		2015			2014
		Derivatives (a)
		(In millions)
	Beginning balance	$	20		$	(1	)
	Total (losses)/gains included in earnings — realized/unrealized	(1		)	2
	Purchases	(3		)	—
	Transfers into Level 3 (b)	—			(1		)
	Ending balance	$	16		$	—
	Losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of March 31	$	(1	)	$	—
	(a) Consists of derivative assets and liabilities, net.
	(b) Transfers in/out of level 3 are related to the availability of external broker quotes and are valued as of the end of the reporting period.
Net counterparty credit exposure by industry sector and by counterparty credit quality	GenOn Americas Generation
	Category	Net Exposure (a)
		(% of Total)
	Financial institutions	75	%
	Utilities, energy merchants, marketers and other	8
	ISOs	17
	Total as of March 31, 2015	100	%
	Category	Net Exposure (a)
		(% of Total)
	Investment grade	99	%
	Non-rated	1
	Total as of March 31, 2015	100	%
	(a) Counterparty credit exposure excludes transportation contracts because of the unavailability of market prices.
GenOn Mid-Atlantic
Fair Value of Financial Instruments
Assets and liabilities measured and recorded at fair value on the consolidated balance sheets on a recurring basis	GenOn Mid-Atlantic
	The following tables present assets and liabilities (including affiliate amounts) measured and recorded at fair value on GenOn Mid-Atlantic's consolidated balance sheet on a recurring basis and their level within the fair value hierarchy:
		As of March 31, 2015
		Fair Value
		Level 1 (a)			Level 2 (a)			Level 3		Total
		(In millions)
	Derivative assets:
	Commodity contracts	$	166		$	190		$	22	$	378
	Derivative liabilities:
	Commodity contracts	$	75		$	115		$	6	$	196
	(a) There were no transfers during the three months ended March 31, 2015, between Levels 1 and 2.
		As of December 31, 2014
		Fair Value
		Level 1 (a)			Level 2 (a)			Level 3		Total
		(In millions)
	Derivative assets:
	Commodity contracts	$	145		$	211		$	26	$	382
	Derivative liabilities:
	Commodity contracts	$	71		$	73		$	6	$	150
	(a) There were no transfers during the year ended December 31, 2014, between Levels 1 and 2.
Reconciliation of beginning and ending balances for financial instruments that are recognized at fair value in the consolidated financial statements at least annually using significant unobservable inputs	The following table reconciles, for the three months ended March 31, 2015, and 2014, the beginning and ending balances for GenOn Mid-Atlantic's derivatives that are recognized at fair value in the consolidated financial statements at least annually using significant unobservable inputs:
		Fair Value Measurement Using Significant Unobservable Inputs (Level 3)
		Three months ended March 31,
		2015			2014
		Derivatives (a)
		(In millions)
	Beginning balance	$	20		$	—
	Total losses included in earnings — realized/unrealized	(1		)	—
	Purchases	(3		)	—
	Ending balance	$	16		$	—
	Losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of March 31	$	(1	)	$	—
	(a) Consists of derivative assets and liabilities, net.
Net counterparty credit exposure by industry sector and by counterparty credit quality	GenOn Mid-Atlantic
	Category	Net Exposure (a)
		(% of Total)
	Financial institutions	100	%
	Category	Net Exposure (a)
		(% of Total)
	Investment grade	100	%
	(a) Counterparty credit exposure excludes transportation contracts because of the unavailability of market prices.

Accounting_for_Derivative_Inst1

Accounting for Derivative Instruments and Hedging Activities (Tables)	3 Months Ended
	Mar. 31, 2015
Accounting for Derivative Instruments and Hedging Activities
Net notional volume buy/(sell) of open derivative transactions broken out by commodity	The following table summarizes the net notional volume buy/(sell) of the Registrants’ open derivative transactions broken out by commodity, excluding those derivatives that qualified for the NPNS exception as of March 31, 2015, and December 31, 2014. Option contracts are reflected using delta volume. Delta volume equals the notional volume of an option adjusted for the probability that the option will be in-the-money at its expiration date.
			GenOn					GenOn Americas Generation					GenOn Mid-Atlantic
			Total Volume					Total Volume					Total Volume
			As of March 31, 2015			As of December 31, 2014		As of March 31, 2015			As of December 31, 2014		As of March 31, 2015			As of December 31, 2014
	Commodity	Units	(In millions)
	Coal	Short Ton	7			8		5			5		5			5
	Natural Gas	MMBtu	85			-21		(38	)		-74		(52	)		-79
	Power	MWh	(41	)		-36		(16	)		-16		(14	)		-15
Fair value within the derivative instrument valuation on the balance sheets	The following tables summarize the fair value within the derivative instrument valuation on the balance sheet:
	GenOn
		Fair Value
		Derivative Assets								Derivative Liabilities
		31-Mar-15				31-Dec-14				31-Mar-15				31-Dec-14
		(In millions)
	Derivatives Not Designated as Cash Flow Hedges:
	Commodity contracts current	$	498			$	602			$	431			$	417
	Commodity contracts long-term	246				205				139				72
	Total Derivatives Not Designated as Cash Flow Hedges	$	744			$	807			$	570			$	489
Offsetting of derivatives by counterparty master agreement level and collateral received or paid	GenOn
			Gross Amounts Not Offset in the Statement of Financial Position
	Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
	December 31, 2014		(In millions)
	Commodity contracts:
	Derivative assets		$	786			$	(425	)		$	(54	)		$	307
	Derivative assets - affiliate		21				(21		)		—				—
	Derivative liabilities		(451		)		425				—				(26		)
	Derivative liabilities - affiliate		(38		)		21				17				—
	Total derivative instruments		$	318			$	—			$	(37	)		$	281
	The following tables summarize the offsetting of derivatives by counterparty master agreement level and collateral received or paid:
	GenOn
			Gross Amounts Not Offset in the Statement of Financial Position
	Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
	March 31, 2015		(In millions)
	Commodity contracts:
	Derivative assets		$	718			$	(466	)		$	(25	)		$	227
	Derivative assets - affiliate		26				(26		)		—				—
	Derivative liabilities		(530		)		466				—				(64		)
	Derivative liabilities - affiliate		(40		)		26				14				—
	Total derivative instruments		$	174			$	—			$	(11	)		$	163
Pre-tax effects of economic hedges that have not been designated as cash flow hedges and trading activity on the statements of operations	The following tables summarize the pre-tax effects of economic hedges that have not been designated as cash flow hedges and trading activity on the Registrants’ statements of operations. These amounts are included within operating revenues and cost of operations.
	GenOn
		Three months ended March 31,
	(In millions)	2015				2014
	Unrealized mark-to-market results
	Reversal of previously recognized unrealized gains on settled positions related to economic hedges	$	(86	)		$	(60	)
	Net unrealized losses on open positions related to economic hedges	(62		)		(171		)
	Total unrealized mark-to-market losses for economic hedging activities	(148		)		(231		)
	Reversal of previously recognized unrealized gains on settled positions related to trading activity	—				(1		)
	Total unrealized mark-to-market losses for trading activity	—				(1		)
	Total unrealized losses	$	(148	)		$	(232	)
		Three months ended March 31,
	(In millions)	2015				2014
	Revenue from operations — energy commodities	$	(99	)		$	(225	)
	Cost of operations	(49		)		(7		)
	Total impact to statements of operations	$	(148	)		$	(232	)
GenOn Americas Generation
Accounting for Derivative Instruments and Hedging Activities
Fair value within the derivative instrument valuation on the balance sheets	GenOn Americas Generation
		Fair Value
		Derivative Assets								Derivative Liabilities
		31-Mar-15				31-Dec-14				31-Mar-15				31-Dec-14
		(In millions)
	Derivatives Not Designated as Cash Flow Hedges:
	Commodity contracts current	$	755			$	852			$	673			$	674
	Commodity contracts long-term	335				256				231				135
	Total Derivatives Not Designated as Cash Flow Hedges	$	1,090			$	1,108			$	904			$	809
Offsetting of derivatives by counterparty master agreement level and collateral received or paid	GenOn Americas Generation
			Gross Amounts Not Offset in the Statement of Financial Position
	Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
	December 31, 2014		(In millions)
	Commodity contracts:
	Derivative assets		$	787			$	(425	)		$	(54	)		$	308
	Derivative assets - affiliate		321				(321		)		—				—
	Derivative liabilities		(451		)		425				—				(26		)
	Derivative liabilities - affiliate		(358		)		321				17				(20		)
	Total derivative instruments		$	299			$	—			$	(37	)		$	262
	GenOn Americas Generation
			Gross Amounts Not Offset in the Statement of Financial Position
	Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
	March 31, 2015		(In millions)
	Commodity contracts:
	Derivative assets		$	718			$	(466	)		$	(25	)		$	227
	Derivative assets - affiliate		372				(372		)		—				—
	Derivative liabilities		(530		)		466				—				(64		)
	Derivative liabilities - affiliate		(374		)		372				2				—
	Total derivative instruments		$	186			$	—			$	(23	)		$	163
Pre-tax effects of economic hedges that have not been designated as cash flow hedges and trading activity on the statements of operations	GenOn Americas Generation
		Three months ended March 31,
	(In millions)	2015				2014
	Unrealized mark-to-market results
	Reversal of previously recognized unrealized gains on settled positions related to economic hedges	$	(95	)		$	(57	)
	Net unrealized losses on open positions related to economic hedges	(22		)		(93		)
	Total unrealized mark-to-market losses for economic hedging activities	(117		)		(150		)
	Reversal of previously recognized unrealized gains on settled positions related to trading activity	—				(1		)
	Total unrealized mark-to-market losses for trading activity	—				(1		)
	Total unrealized losses	$	(117	)		$	(151	)
		Three months ended March 31,
	(In millions)	2015				2014
	Revenue from operations — energy commodities	$	(86	)		$	(145	)
	Cost of operations	(31		)		(6		)
	Total impact to statements of operations	$	(117	)		$	(151	)
GenOn Mid-Atlantic
Accounting for Derivative Instruments and Hedging Activities
Fair value within the derivative instrument valuation on the balance sheets	GenOn Mid-Atlantic
		Fair Value
		Derivative Assets								Derivative Liabilities
		31-Mar-15				31-Dec-14				31-Mar-15				31-Dec-14
		(In millions)
	Derivatives Not Designated as Cash Flow Hedges:
	Commodity contracts current	$	228			$	241			$	148			$	128
	Commodity contracts long-term	150				141				48				22
	Total Derivatives Not Designated as Cash Flow Hedges	$	378			$	382			$	196			$	150
Offsetting of derivatives by counterparty master agreement level and collateral received or paid	GenOn Mid-Atlantic
			Gross Amounts Not Offset in the Statement of Financial Position
	Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
	March 31, 2015		(In millions)
	Commodity contracts:
	Derivative assets		$	83			$	(1	)		$	—			$	82
	Derivative assets - affiliate		295				(195		)		—				100
	Derivative liabilities		(1		)		1				—				—
	Derivative liabilities - affiliate		(195		)		195				—				—
	Total derivative instruments		$	182			$	—			$	—			$	182
	GenOn Mid-Atlantic
			Gross Amounts Not Offset in the Statement of Financial Position
	Description		Gross Amounts of Recognized Assets / Liabilities				Derivative Instruments				Cash Collateral (Held) / Posted				Net Amount
	December 31, 2014		(In millions)
	Commodity contracts:
	Derivative assets		$	100			$	—			$	—			$	100
	Derivative assets - affiliate		282				(149		)		—				133
	Derivative liabilities		(1		)		—				—				(1		)
	Derivative liabilities - affiliate		(149		)		149				—				—
	Total derivative instruments		$	232			$	—			$	—			$	232
Pre-tax effects of economic hedges that have not been designated as cash flow hedges and trading activity on the statements of operations	GenOn Mid-Atlantic
		Three months ended March 31,
	(In millions)	2015				2014
	Unrealized mark-to-market results
	Reversal of previously recognized unrealized gains on settled positions related to economic hedges	$	(27	)		$	(63	)
	Net unrealized losses on open positions related to economic hedges	(28		)		(92		)
	Total unrealized losses	$	(55	)		$	(155	)
		Three months ended March 31,
	(In millions)	2015				2014
	Revenue from operations — energy commodities	$	(24	)		$	(150	)
	Cost of operations	(31		)		(5		)
	Total impact to statements of operations	$	(55	)		$	(155	)

Income_Taxes_Tables

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income tax (benefit)/provision	GenOn’s income tax expense/(benefit) consisted of the following:
		Three months ended March 31,
	(In millions except otherwise noted)	2015			2014
	(Loss)/Income before income taxes	$	(12	)	$	4
	Income tax (benefit)/expense	(1		)	1
	Effective tax rate	8.3		%	25		%

Related_Party_Transactions_Tab

Related Party Transactions (Tables)	3 Months Ended
	Mar. 31, 2015
GenOn Americas Generation
Related Party Transaction
Summary of material related-party transactions with affiliates	The following costs were incurred under these arrangements:
	GenOn Americas Generation
		Three months ended March 31,
		2015		2014
		(In millions)
	Allocated costs:
	Cost of operations — affiliate	$	—	$	2
	Selling, general and administrative — affiliate	21		18
	Total	$	21	$	20
GenOn Mid-Atlantic
Related Party Transaction
Summary of material related-party transactions with affiliates	GenOn Mid-Atlantic
		Three months ended March 31,
		2015		2014
		(In millions)
	Allocated costs:
	Cost of operations — affiliate	$	—	$	1
	Selling, general and administrative — affiliate	15		15
	Total	$	15	$	16

Basis_of_Presentation_Details

Basis of Presentation (Details)	3 Months Ended
	Mar. 31, 2015
	MW
Basis of Presentation
Generation capacity (in MW)	18,430
Change in Power Generation Capacity, Megawatts	1,099
GenOn Mid-Atlantic
Basis of Presentation
Generation capacity (in MW)	4,683
GenOn Americas Generation
Basis of Presentation
Generation capacity (in MW)	7,596

Summary_of_Significant_Account1

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Summary of Significant Accounting Policies [Line Items]
Inventory Write-down	$11

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Carrying Amount
Fair Value of Financial Instruments
Long-term debt	$3,107	$3,122
Fair Value
Fair Value of Financial Instruments
Long-term debt	2,829	2,706
GenOn Americas Generation | Carrying Amount
Fair Value of Financial Instruments
Long-term debt	926	929
GenOn Americas Generation | Fair Value
Fair Value of Financial Instruments
Long-term debt	$796	$720

Fair_Value_of_Financial_Instru3

Fair Value of Financial Instruments (Details 2) (Fair Value, Measurements, Recurring, USD $)	Mar. 31, 2015		Dec. 31, 2014
In Millions, unless otherwise specified
Level 1
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	$20	[1],[2]	$21	[2],[3]
Level 1 | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	197	[1]	179	[3]
Derivative liabilities	118	[1]	105	[3]
Level 2
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0	[1],[2]	0	[2]
Level 2 | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	516	[1]	582
Derivative liabilities	437	[1]	371
Fair Value, Inputs, Level 3 [Member]
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	0	[2]	0	[2]
Fair Value, Inputs, Level 3 [Member] | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	31		46
Derivative liabilities	15		13
Fair Value
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Other assets	20	[2]	21	[2]
Fair Value | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	744		807
Derivative liabilities	570		489
GenOn Americas Generation | Level 1 | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	235	[1]	208	[3]
Derivative liabilities	146	[1]	137	[3]
GenOn Americas Generation | Level 2 | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	816	[1]	848	[3]
Derivative liabilities	735	[1]	640	[3]
GenOn Americas Generation | Fair Value, Inputs, Level 3 [Member] | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	39		52
Derivative liabilities	23		32
GenOn Americas Generation | Fair Value | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,090		1,108
Derivative liabilities	904		809
GenOn Mid-Atlantic | Level 1 | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	166	[1]	145	[3]
Derivative liabilities	75	[1]	71	[3]
GenOn Mid-Atlantic | Level 2 | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	190	[1]	211	[3]
Derivative liabilities	115	[1]	73	[3]
GenOn Mid-Atlantic | Fair Value, Inputs, Level 3 [Member] | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	22		26
Derivative liabilities	6		6
GenOn Mid-Atlantic | Fair Value | Commodity contracts
Fair Value Assets and Liabilities, Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	378		382
Derivative liabilities	$196		$150
[1]	There were no transfers during the three months ended March 31, 2015, between Levels 1 and 2.
[2]	Relates to mutual funds held in a rabbi trust for non-qualified deferred compensation plans for certain key and highly compensated employees
[3]	There were no transfers during the year ended December 31, 2014, between Levels 1 and 2.

Fair_Value_of_Financial_Instru4

Fair Value of Financial Instruments (Details 3) (Fair Value, Inputs, Level 3 [Member], Derivative [Member], USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014		Dec. 31, 2013
Fair Value Asset and Liabilities, Measured on Recurring Basis, Unobservable Input Changes
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$16	[1]	($2)	[1]	$33	[1]	($4)	[1]
Reconciliation of the beginning and ending balances for financial instruments that are recognized at fair value in the consolidated financial statements
Total gains and losses (realized/unrealized) included in earnings	-13	[1]	4	[1]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases	-4	[1]	0	[1]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset and Liability Transfers into Level 3	0	[1],[2]	-2	[1],[2]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings	-7	[1]	0	[1]
GenOn Americas Generation
Fair Value Asset and Liabilities, Measured on Recurring Basis, Unobservable Input Changes
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	16	[1]	0	[1]	20	[1]	-1	[1]
Reconciliation of the beginning and ending balances for financial instruments that are recognized at fair value in the consolidated financial statements
Total gains and losses (realized/unrealized) included in earnings	-1	[1]	2	[1]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases	-3	[1]	0	[1]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset and Liability Transfers into Level 3	0	[1],[2]	-1	[1],[2]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings	-1	[1]	0	[1]
GenOn Mid-Atlantic
Fair Value Asset and Liabilities, Measured on Recurring Basis, Unobservable Input Changes
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	16	[3]	0	[3]	20	[3]	0	[3]
Reconciliation of the beginning and ending balances for financial instruments that are recognized at fair value in the consolidated financial statements
Total gains and losses (realized/unrealized) included in earnings	-1	[3]	0	[3]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases	-3	[3]	0	[3]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings	($1)	[3]	$0	[3]
[1]	Consists of derivative assets and liabilities, net.
[2]	Transfers in/out of level 3 are related to the availability of external broker quotes and are valued as of the end of the reporting period.
[3]	Consists of derivative assets and liabilities, net

Fair_Value_of_Financial_Instru5

Fair Value of Financial Instruments (Details 4) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Derivative fair value measurements
Total derivative assets valued with prices provied by models and other valuation techniques (as a percent)	4.00%
Total derivative liabilities valued with prices provied by models and other valuation techniques (as a percent)	3.00%
Credit reserve balance	$0		$0
Concentration of credit risk
Counterparty credit exposure	361
Collateral held (cash and letters of credit) against counterparty credit exposure	4
Counterparty credit exposure, net	361
Counterparty credit exposure before collateral expected to roll off by the end of 2014 (as a percent)	83.00%
Net exposure (as a percent)	100.00%	[1]
Fair Value Inputs, Counterparty Credit Risk	10.00%
Aggregate counterparty credit risk exposure for counterparties representing exposure above threshold percentage	285
Investment grade
Concentration of credit risk
Net exposure (as a percent)	97.00%	[1]
External Credit Rating, Non Investment Grade [Member]
Concentration of credit risk
Net exposure (as a percent)	2.00%	[1]
Non-rated
Concentration of credit risk
Net exposure (as a percent)	1.00%	[1]
Financial institutions
Concentration of credit risk
Net exposure (as a percent)	72.00%	[1]
Utilities, energy merchants, marketers and other
Concentration of credit risk
Net exposure (as a percent)	12.00%	[1]
ISOs
Concentration of credit risk
Net exposure (as a percent)	16.00%	[1]
GenOn Americas Generation
Derivative fair value measurements
Total derivative assets valued with prices provied by models and other valuation techniques (as a percent)	4.00%
Total derivative liabilities valued with prices provied by models and other valuation techniques (as a percent)	3.00%
Credit reserve balance	0		0
Concentration of credit risk
Counterparty credit exposure	347
Collateral held (cash and letters of credit) against counterparty credit exposure	4
Counterparty credit exposure, net	347
Counterparty credit exposure before collateral expected to roll off by the end of 2014 (as a percent)	85.00%
Net exposure (as a percent)	100.00%	[1]
Fair Value Inputs, Counterparty Credit Risk	10.00%
Aggregate counterparty credit risk exposure for counterparties representing exposure above threshold percentage	285
GenOn Americas Generation | Investment grade
Concentration of credit risk
Net exposure (as a percent)	99.00%	[1]
GenOn Americas Generation | Non-rated
Concentration of credit risk
Net exposure (as a percent)	1.00%	[1]
GenOn Americas Generation | Financial institutions
Concentration of credit risk
Net exposure (as a percent)	75.00%	[1]
GenOn Americas Generation | Utilities, energy merchants, marketers and other
Concentration of credit risk
Net exposure (as a percent)	8.00%	[1]
GenOn Americas Generation | ISOs
Concentration of credit risk
Net exposure (as a percent)	17.00%	[1]
GenOn Mid-Atlantic
Derivative fair value measurements
Total derivative assets valued with prices provied by models and other valuation techniques (as a percent)	6.00%
Total derivative liabilities valued with prices provied by models and other valuation techniques (as a percent)	3.00%
Credit reserve balance	2		2
Concentration of credit risk
Counterparty credit exposure	83
Collateral held (cash and letters of credit) against counterparty credit exposure	0
Counterparty credit exposure, net	83
Counterparty credit exposure before collateral expected to roll off by the end of 2014 (as a percent)	100.00%
Fair Value Inputs, Counterparty Credit Risk	10.00%
Aggregate counterparty credit risk exposure for counterparties representing exposure above threshold percentage	$83
GenOn Mid-Atlantic | Investment grade
Concentration of credit risk
Net exposure (as a percent)	100.00%	[1]
GenOn Mid-Atlantic | Financial institutions
Concentration of credit risk
Net exposure (as a percent)	100.00%	[1]
[1]	Counterparty credit exposure excludes transportation contracts because of the unavailability of market prices.

Accounting_for_Derivative_Inst2

Accounting for Derivative Instruments and Hedging Activities Disclosure Accounting for Derivative Instruments and Hedging Activities (Details 1)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
	T	T
Coal | Short Ton [Member]
Derivative
Derivative, Nonmonetary Notional Amount, Mass	7,000,000	8,000,000
Coal | Short Ton [Member] | GenOn Americas Generation
Derivative
Derivative, Nonmonetary Notional Amount, Mass	5,000,000	5,000,000
Coal | Short Ton [Member] | GenOn Mid-Atlantic
Derivative
Derivative, Nonmonetary Notional Amount, Mass	5,000,000	5,000,000
Natural Gas [Member] | MMbtu [Member]
Derivative
Derivative, Nonmonetary Notional Amount, Energy Measure	85,000,000	-21,000,000
Natural Gas [Member] | MMbtu [Member] | GenOn Americas Generation
Derivative
Derivative, Nonmonetary Notional Amount, Energy Measure	-38,000,000	-74,000,000
Natural Gas [Member] | MMbtu [Member] | GenOn Mid-Atlantic
Derivative
Derivative, Nonmonetary Notional Amount, Energy Measure	-52,000,000	-79,000,000
Power [Member] | M Wh [Member]
Derivative
Derivative, Nonmonetary Notional Amount, Energy Measure	-41,000,000	-36,000,000
Power [Member] | M Wh [Member] | GenOn Americas Generation
Derivative
Derivative, Nonmonetary Notional Amount, Energy Measure	-16,000,000	-16,000,000
Power [Member] | M Wh [Member] | GenOn Mid-Atlantic
Derivative
Derivative, Nonmonetary Notional Amount, Energy Measure	-14,000,000	-15,000,000

Accounting_for_Derivative_Inst3

Accounting for Derivative Instruments and Hedging Activities Disclosure Accounting for Derivative Instruments and Hedging Activities (Details 2) (Not Designated as Hedging Instrument [Member], USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Derivative
Derivative Asset, Fair Value, Gross Asset	$744	$807
Derivative Liability, Fair Value, Gross Liability	570	489
GenOn Americas Generation
Derivative
Derivative Asset, Fair Value, Gross Asset	1,090	1,108
Derivative Liability, Fair Value, Gross Liability	904	809
GenOn Mid-Atlantic
Derivative
Derivative Asset, Fair Value, Gross Asset	378	382
Derivative Liability, Fair Value, Gross Liability	196	150
Commodity Contract Current [Member]
Derivative
Derivative Asset, Fair Value, Gross Asset	498	602
Derivative Liability, Fair Value, Gross Liability	431	417
Commodity Contract Current [Member] | GenOn Americas Generation
Derivative
Derivative Asset, Fair Value, Gross Asset	755	852
Derivative Liability, Fair Value, Gross Liability	673	674
Commodity Contract Current [Member] | GenOn Mid-Atlantic
Derivative
Derivative Asset, Fair Value, Gross Asset	228	241
Derivative Liability, Fair Value, Gross Liability	148	128
Commodity Contract Non Current [Member]
Derivative
Derivative Asset, Fair Value, Gross Asset	246	205
Derivative Liability, Fair Value, Gross Liability	139	72
Commodity Contract Non Current [Member] | GenOn Americas Generation
Derivative
Derivative Asset, Fair Value, Gross Asset	335	256
Derivative Liability, Fair Value, Gross Liability	231	135
Commodity Contract Non Current [Member] | GenOn Mid-Atlantic
Derivative
Derivative Asset, Fair Value, Gross Asset	150	141
Derivative Liability, Fair Value, Gross Liability	$48	$22

Accounting_for_Derivative_Inst4

Accounting for Derivative Instruments and Hedging Activities (Details 3) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Fair Value of Derivative Instruments/Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Cash Collateral (Held)	($29)	($54)
Cash Collateral Posted	-100	-38
Gross Amounts of Recognized Assets / Liabilities	174	318
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	-11	-37
Net Amount	163	281
GenOn Americas Generation
Fair Value of Derivative Instruments/Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Cash Collateral (Held)	-29	-54
Cash Collateral Posted	-91	-29
Gross Amounts of Recognized Assets / Liabilities	186	299
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	-23	-37
Net Amount	163	262
GenOn Mid-Atlantic
Fair Value of Derivative Instruments/Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Gross Amounts of Recognized Assets / Liabilities	182	232
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	0	0
Net Amount	182	232
Non-affiliate | Commodity contracts
Fair Value of Derivative Instruments/Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Gross Amount of Recognized Derivative Assets	718	786
Gross Amounts of Recognized Derivative Liabilities	-530	-451
Derivative Instruments	-466	-425
Derivative Instruments	466	425
Cash Collateral (Held)	-25	-54
Cash Collateral Posted	0	0
Net Amount	227	307
Net Amount	-64	-26
Non-affiliate | GenOn Americas Generation | Commodity contracts
Fair Value of Derivative Instruments/Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Gross Amount of Recognized Derivative Assets	718	787
Gross Amounts of Recognized Derivative Liabilities	-530	-451
Derivative Instruments	-466	-425
Derivative Instruments	466	425
Cash Collateral (Held)	-25	-54
Cash Collateral Posted	0	0
Net Amount	227	308
Net Amount	-64	-26
Non-affiliate | GenOn Mid-Atlantic | Commodity contracts
Fair Value of Derivative Instruments/Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Gross Amount of Recognized Derivative Assets	83	100
Gross Amounts of Recognized Derivative Liabilities	-1	-1
Derivative Instruments	-1	0
Derivative Instruments	1	0
Cash Collateral (Held)	0	0
Cash Collateral Posted	0	0
Net Amount	82	100
Net Amount	0	-1
Affiliate | Commodity contracts
Fair Value of Derivative Instruments/Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Gross Amount of Recognized Derivative Assets	26	21
Gross Amounts of Recognized Derivative Liabilities	-40	-38
Derivative Instruments	-26	-21
Derivative Instruments	26	21
Cash Collateral (Held)	0	0
Cash Collateral Posted	14	17
Net Amount	0	0
Net Amount	0	0
Affiliate | GenOn Americas Generation | Commodity contracts
Fair Value of Derivative Instruments/Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Gross Amount of Recognized Derivative Assets	372	321
Gross Amounts of Recognized Derivative Liabilities	-374	-358
Derivative Instruments	-372	-321
Derivative Instruments	372	321
Cash Collateral (Held)	0	0
Cash Collateral Posted	2	17
Net Amount	0	0
Net Amount	0	-20
Affiliate | GenOn Mid-Atlantic | Commodity contracts
Fair Value of Derivative Instruments/Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Gross Amount of Recognized Derivative Assets	295	282
Gross Amounts of Recognized Derivative Liabilities	-195	-149
Derivative Instruments	-195	-149
Derivative Instruments	195	149
Cash Collateral (Held)	0	0
Cash Collateral Posted	0	0
Net Amount	100	133
Net Amount	$0	$0

Accounting_for_Derivative_Inst5

Accounting for Derivative Instruments and Hedging Activities (Details 5) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Unrealized mark-to-market results
Reversal of previously recognized unrealized losses/(gains) on settled positions related to economic hedges	($86)	($60)
Net unrealized gains/(losses) on open positions related to economic hedges	62	171
Total unrealized mark-to-market gains/(losses) for economic hedging activities	148	231
Reversal of previously recognized unrealized losses/(gains) on settled positions related to trading activity	0	-1
Total unrealized mark-to-market gains/(losses) for trading activity	0	1
Total unrealized gains/(losses)	-148	-232
Impact of derivative instruments to statement of operations
Total impact to statement of operations	148	232
Credit Risk Related Contingent Features
Collateral required for contracts with adequate assurance clauses in net liability positions	39
Collateral required for contracts with credit rating contingent features in net liability position	1
Collateral due on net liability position that has not been called by a certain marginable agreement counterparty	0
Gains (losses) included in operating revenues
Unrealized mark-to-market results
Total unrealized gains/(losses)	-99	-225
Impact of derivative instruments to statement of operations
Total impact to statement of operations	99	225
Gains (losses) included in cost of operations
Unrealized mark-to-market results
Total unrealized gains/(losses)	-49	-7
Impact of derivative instruments to statement of operations
Total impact to statement of operations	49	7
GenOn Americas Generation
Unrealized mark-to-market results
Reversal of previously recognized unrealized losses/(gains) on settled positions related to economic hedges	-95	-57
Net unrealized gains/(losses) on open positions related to economic hedges	22	93
Total unrealized mark-to-market gains/(losses) for economic hedging activities	117	150
Reversal of previously recognized unrealized losses/(gains) on settled positions related to trading activity	0	-1
Total unrealized mark-to-market gains/(losses) for trading activity	0	1
Total unrealized gains/(losses)	-117	-151
Impact of derivative instruments to statement of operations
Total impact to statement of operations	117	151
Credit Risk Related Contingent Features
Collateral required for contracts with adequate assurance clauses in net liability positions	39
Collateral required for contracts with credit rating contingent features in net liability position	1
Collateral due on net liability position that has not been called by a certain marginable agreement counterparty	0
GenOn Americas Generation | Gains (losses) included in operating revenues
Unrealized mark-to-market results
Total unrealized gains/(losses)	-86	-145
Impact of derivative instruments to statement of operations
Total impact to statement of operations	86	145
GenOn Americas Generation | Gains (losses) included in cost of operations
Unrealized mark-to-market results
Total unrealized gains/(losses)	-31	-6
Impact of derivative instruments to statement of operations
Total impact to statement of operations	31	6
GenOn Mid-Atlantic
Unrealized mark-to-market results
Reversal of previously recognized unrealized losses/(gains) on settled positions related to economic hedges	-27	-63
Net unrealized gains/(losses) on open positions related to economic hedges	28	92
Total unrealized gains/(losses)	-55	-155
Impact of derivative instruments to statement of operations
Total impact to statement of operations	55	155
GenOn Mid-Atlantic | Gains (losses) included in operating revenues
Unrealized mark-to-market results
Total unrealized gains/(losses)	-24	-150
Impact of derivative instruments to statement of operations
Total impact to statement of operations	24	150
GenOn Mid-Atlantic | Gains (losses) included in cost of operations
Unrealized mark-to-market results
Total unrealized gains/(losses)	-31	-5
Impact of derivative instruments to statement of operations
Total impact to statement of operations	$31	$5

Income_Taxes_Details

Income Taxes (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Income tax provision
Income/(loss) before income taxes	($12)	$4
Income tax (benefit)/expense	-1	1
Effective tax rate (as a percent)	8.30%	25.00%
Statutory tax rate (as a percent)	35.00%
GenOn Americas Generation
Income tax provision
Income/(loss) before income taxes	-7	65
Income tax (benefit)/expense	0	0
GenOn Americas Generation [Member]
Income tax provision
Income tax (benefit)/expense	0	0
GenOn Mid-Atlantic
Income tax provision
Income/(loss) before income taxes	23	43
Income tax (benefit)/expense	0	0
Pro Forma | GenOn Americas Generation [Member]
Income tax provision
Income tax (benefit)/expense	0	0
Pro Forma | GenOn Mid-Atlantic
Income tax provision
Income tax (benefit)/expense	8	17
Deferred income taxes	$43		$51

Related_Party_Transactions_Det

Related Party Transactions (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 14, 2012
GenOn Americas Generation | GenOn
Related Party Transaction
Affiliate cost recorded in cost of operations	$21	$20
GenOn Americas Generation | GenOn Energy Holdings
Related Party Transaction
Current note receivable from affiliate	331		331
GenOn Mid-Atlantic | GenOn
Related Party Transaction
Affiliate cost recorded in cost of operations	15	16
Direct cost of operations | GenOn Americas Generation | GenOn
Related Party Transaction
Affiliate cost recorded in cost of operations	47	33
Intercompany Credit Agreement
Related Party Transaction
Line of Credit Facility, Maximum Borrowing Capacity				500
Line of Credit Facility, Maximum Borrowing Capacity	0		0
Letter of Credit | Intercompany Credit Agreement | NRG Energy
Related Party Transaction
Term of Facility	3 years
Debt instrument, interest rate over variable rate (as a percent)	2.50%
Basis points added to interest under LIBOR rate loans (as a percent)	3.50%
Letter of Credit | Intercompany Credit Agreement | GenOn Americas Generation | NRG Energy
Related Party Transaction
Letters of credit issued and oustanding on behalf of entity by affiliate	163		173
Letter of Credit | Intercompany Credit Agreement | GenOn Mid-Atlantic | NRG Energy
Related Party Transaction
Letters of credit issued and oustanding on behalf of entity by affiliate	240		237
Annual [Domain] | NRG Energy
Related Party Transaction
Service Management Costs	$193

Related_Party_Transactions_Rel

Related Party Transactions Related Party Transactions (Details 2) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
GenOn Mid-Atlantic | GenOn
Related Party Transaction
Related Party Transaction, Expenses from Transactions with Related Party	$15	$16
GenOn Mid-Atlantic | Allocated cost of operations [Member] | GenOn
Related Party Transaction
Related Party Transaction, Expenses from Transactions with Related Party	0	1
GenOn Mid-Atlantic | Allocated Selling, General and Administrative [Member] | GenOn
Related Party Transaction
Related Party Transaction, Selling, General and Administrative Expenses from Transactions with Related Party	15	15
GenOn Americas Generation
Related Party Transaction
Due from Affiliate, Current	35		0
GenOn Americas Generation | GenOn
Related Party Transaction
Related Party Transaction, Expenses from Transactions with Related Party	21	20
GenOn Americas Generation | GenOn Energy Holdings [Member]
Related Party Transaction
Due from Affiliate, Current	165		118
Notes Receivable, Related Parties, Current	331		331
GenOn Americas Generation | Allocated cost of operations [Member] | GenOn
Related Party Transaction
Related Party Transaction, Expenses from Transactions with Related Party	0	2
GenOn Americas Generation | Allocated Selling, General and Administrative [Member] | GenOn
Related Party Transaction
Related Party Transaction, Selling, General and Administrative Expenses from Transactions with Related Party	$21	$18

Commitments_and_Contingencies_

Commitments and Contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2008	Mar. 31, 2014	Dec. 14, 2012	Dec. 02, 2010
MDE v. GenOn Chalk Point and GenOn Mid-Atlantic | GenOn Mid-Atlantic | Environmental Matters
Maryland Department of the Environment v. GenOn Chalk Point and GenOn Mid-Atlantic
Loss Contingency, Damages Sought, Value	$100,000
Chapter Eleven Proceedings | GenOn Americas Generation | Bankruptcy
Chapter Eleven Proceedings
GenOn Energy Holdings common stock shares reserved for unresolved claims		461,000
Number of reserved shares for disputed Chapter 11 claims converted into reserved of GenOn common stock				1,300,000
Number of reserved shares for disputed Chapter 11 claims converted into reserved of NRG common stock			159,000

Regulatory_Matters_Regulatory_

Regulatory Matters Regulatory Matters (Details) (GenOn, USD $)	Dec. 20, 2013
In Millions, unless otherwise specified
GenOn
Regulatory Matters [Line Items]
Regulatory payments sought	$22

Environmental_Matters_Details

Environmental Matters (Details) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified
Environmental Matters Disclosure
Estimated environmental capital expenditures from 2013 through 2017	$57
GenOn Americas Generation
Environmental Matters Disclosure
Estimated environmental capital expenditures from 2013 through 2017	19
GenOn Mid-Atlantic
Environmental Matters Disclosure
Estimated environmental capital expenditures from 2013 through 2017	$13