Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2013
Accounting Policies [Abstract]
Cash Equivalents	Cash Equivalents Cash equivalents included money market funds totaling $983 million and $615 million for EME and $101 million and $75 million for Midwest Generation at December 31, 2013 and 2012, respectively. The carrying value of cash equivalents equals the fair value as all investments have original maturities of less than three months.
Restricted Cash and Cash Equivalents, and Restricted Deposits	Restricted Cash and Cash Equivalents, and Restricted Deposits Restricted deposits consisted of cash balances that are restricted to pay amounts required for lease payments, debt service or to provide collateral. At December 31, 2013 and 2012, EME's restricted cash and deposits included $22 million and $49 million, respectively, to support letters of credit issued under EME's letter of credit facilities. Restricted deposits of $4 million as of both December 31, 2013 and 2012 were included in other long-term assets on Midwest Generation's consolidated balance sheets. These cash balances are restricted to provide collateral or other deposits required by contract.
Inventory	Inventory Inventory is stated at the lower of weighted-average cost or market. Inventory is recorded at actual cost when purchased and then expensed at weighted-average cost as used.
Variable Interest Entities	VIEs (EME only) A VIE is a legal entity whose equity owners do not have sufficient equity at risk, or as a group, the holders of the equity investment at risk lack any of the following three characteristics: decision making rights, the obligation to absorb losses, or the right to receive the residual returns of the entity. The primary beneficiary is identified as the variable interest holder that has both the power to direct the activities of the VIE that most significantly impact the entity's economic performance and the obligation to absorb losses or the right to receive benefits from the entity that could potentially be significant to the VIE. The primary beneficiary is required to consolidate the VIE unless specific exceptions or exclusions are met. Commercial and operating activities are generally the factors that most significantly impact the economic performance of VIEs in which EME has a variable interest. Commercial and operating activities include construction, operation and maintenance, fuel procurement, dispatch and compliance with regulatory and contractual requirements. Allocation of Net Income or Losses to Investors in Certain VIEs (EME only) During 2012 and 2013, EME raised third-party capital to support the development of a portion of EME's wind portfolio by selling indirect equity interests in certain wind projects through a new venture, Capistrano Wind Partners. Capistrano Wind Partners' partnership agreements contain complex allocation provisions for taxable income and losses, tax credits and cash distributions. EME allocates net income for this consolidated investment to third-party investors based on the Hypothetical Liquidation Book Value (HLBV) method. HLBV is a balance sheet oriented approach that calculates the change in the claims of each partner on the net assets of the investment at the beginning and end of each period. Each partner's claim is equal to the amount each party would receive or pay if the net assets of the investment were to liquidate at book value and the resulting cash was then distributed to investors in accordance with their respective liquidation preferences. EME reports the net income (loss) attributable to the third-party investors as (income) loss attributable to noncontrolling interests in the consolidated statements of operations. For further information, see Note 3—Variable Interest Entities—Categories of VIEs—Capistrano Wind Equity Capital.
Purchased Emission Allowances, Exemptions and Offsets	Purchased Emission Allowances, Exemptions and Offsets (EME only) Purchased emission allowances are stated at the lower of weighted-average cost or market. Purchased emission allowances are recorded at cost when purchased and then expensed at weighted-average cost as used. Cost is reduced to market value if the market value of emission allowances has declined and it is probable that revenues earned from the generation of power will not cover the amounts recorded in the ordinary course of business. Purchased emission allowances are classified as current or long-term assets based on the time the allowances are expected to be used. At December 31, 2013 and 2012, EME had $4 million and $16 million, respectively, of purchased emission allowances, exemptions and offsets, primarily related to the Walnut Creek facility, reflected in other long-term assets in the accompanying consolidated balance sheets.
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment, including leasehold improvements and construction in progress, are capitalized at cost. Depreciation and amortization are computed using the straight-line method over the estimated useful life of the property, plant and equipment and over the shorter of the lease term or estimated useful life for leasehold improvements. The costs of repairs and maintenance, including periodic major maintenance, are expensed as incurred. As part of the acquisition of the Midwest Generation plants, EME acquired emission allowances under the United States Environmental Protection Agency's (US EPA) Acid Rain Program. EME uses these emission allowances in the normal course of its business to generate electricity and has classified them as part of property, plant and equipment. Acquired emission allowances are amortized on a straight-line basis. The remaining estimated useful life or lease term at December 31, 2013 for the Midwest Generation plants is as follows. Estimated useful lives of individual facilities could be impacted by decisions related to the installation of environmental remediation equipment. If environmental compliance equipment is not installed, the useful life may be shortened. Interest incurred on funds borrowed by EME is capitalized during the construction period. Such capitalized interest is included in property, plant and equipment. Capitalized interest is amortized over the depreciation period of the major plant and facilities for the respective project. Capitalized interest was $7 million, $31 million and $27 million in 2013, 2012 and 2011, respectively. Midwest Generation did not record capitalized interest during the period.
Asset Retirement Obligations	Asset Retirement Obligations Authoritative guidance on asset retirement obligations (AROs) requires entities to record the fair value of a liability for an ARO in the period in which it is incurred, including a liability for the fair value of a conditional ARO, if the fair value can be reasonably estimated even though uncertainty exists about the timing and/or method of settlement. When an ARO liability is initially recorded, the entity capitalizes the cost by increasing the carrying amount of the related long-lived asset. Over time, the liability is increased for accretion expense to its present value each period, and the capitalized cost is depreciated over the useful life of the related asset. Settlement of an ARO liability for an amount other than its recorded amount results in an increase or decrease in expense. EME and Midwest Generation have recorded a liability representing expected future costs associated with site reclamations, facilities dismantlement and removal of environmental hazards, which is included in other long-term liabilities on EME's consolidated balance sheets and benefit plans and other long-term liabilities on Midwest Generation's consolidated balance sheets.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets EME and Midwest Generation evaluate the impairment of long-lived assets based on a review of estimated future cash flows expected to be generated whenever events or changes in circumstances indicate that the carrying amount of such investments or assets may not be recoverable. EME's and Midwest Generation's unit of account is at the plant level and, accordingly, the closure of a unit at a multi-unit site would not result in an impairment of property, plant and equipment unless such condition were to affect an impairment assessment on the entire plant. If the carrying amount of a long-lived asset exceeds the expected future cash flows, undiscounted and without interest charges, an impairment loss is recognized for the excess of the carrying amount over fair value. Fair value is determined via market, cost and income based valuation techniques, as appropriate. For further discussion, see Note 13—Asset Impairments and Other Charges. EME also evaluates investments in unconsolidated affiliates for potential impairment. If the carrying value of an unconsolidated affiliate exceeds its fair value, an impairment loss is recorded if the decline is other than temporary.
Sale Leasebacks	EME EME accounts for long-term leases associated with the Powerton and Joliet Sale Leaseback as operating leases on its separate consolidated financial statements. Minimum lease payments under operating leases are levelized (total minimum lease payments divided by the number of years of the lease) and recorded as rent expense over the terms of the leases. Lease payments in excess of the minimum are recorded as rent expense in the year incurred. Midwest Generation Midwest Generation accounts for the Powerton and Joliet Sale Leaseback as a lease financing in its separate consolidated financial statements. Accordingly, Midwest Generation records the power plants as assets in a similar manner to a capital lease and records depreciation expense from the power plants and interest expense from the lease financing.
Allowance for Losses on Notes Receivable	Allowance for Losses on Notes Receivable (Midwest Generation only) Notes receivable are evaluated for impairment whenever events or changes in circumstances indicate that the carrying amount of such assets may not be recoverable. A valuation allowance will be recorded when it is probable that Midwest Generation will be unable to collect amounts due, including principal and interest, according to the contractual terms and schedule of the loan agreement. For additional information on Midwest Generation's impaired intercompany loan, see Note 15—Related Party Transactions.
Accounting for Reorganization	Accounting for Reorganization As a result of the EME and Midwest Generation Chapter 11 Cases, realization of assets and satisfaction of liabilities are subject to a significant number of uncertainties. The consolidated financial statements prepared under Accounting Standards Codification (ASC) 852 Reorganizations require the following accounting policies for debtors-in-possession. Liabilities Subject to Compromise (LSTC) Unsecured prepetition liabilities that have at least a possibility of not being fully repaid have been reclassified into LSTC, a separate line item on the consolidated balance sheet. LSTC, including claims that have become known after the bankruptcy filing, are reported on the basis of the probably allowed claim. For additional information, see Note 16—Restructuring Activities. Reorganization Items Adjustments to amounts classified as LSTC are presented as Reorganization Items, a separate line item on the consolidated statement of operations. Reorganization items include direct and incremental costs of bankruptcy, such as professional fees. In 2012, reorganization items also included the write off of deferred financing costs of $15 million related to the classification of EME's senior notes as part of LSTC. For additional information, see Note 16—Restructuring Activities. Interest Expense EME and Midwest Generation will not pay interest expense during bankruptcy and it is not expected to be an allowable claim. Therefore, the filing entities will not accrue interest expense for financial reporting purposes; however, unpaid contractual interest is calculated for disclosure purposes.
Deferred Financing Costs	Deferred Financing Costs (EME only) Bank, legal and other direct costs incurred in connection with obtaining financing are deferred and amortized as interest expense on a basis that approximates the effective interest rate method over the term of the related debt. Amortization of deferred financing costs charged to interest expense was $13 million, $19 million and $15 million in 2013, 2012 and 2011, respectively. For additional information, see "Reorganization Items" above.
Revenue Recognition	Revenue Recognition Generally, revenues and related costs are recognized when electricity is generated, or services are provided, unless the transaction is accounted for as a derivative and does not qualify for the normal purchases and sales exception. EME's subsidiaries enter into power and fuel hedging, optimization transactions and energy trading contracts, all subject to market conditions. One of EME's subsidiaries executes these transactions primarily through the use of physical forward commodity purchases and sales and financial commodity swaps and options. With respect to its physical forward contracts, EME's subsidiaries generally act as the principal, take title to the commodities, and assume the risks and rewards of ownership. EME's subsidiaries record the settlement of non-trading physical forward contracts on a gross basis. EME nets the cost of purchased power against related third-party sales in markets that use locational marginal pricing, currently PJM. Financial swap and option transactions are settled net and, accordingly, EME's subsidiaries do not take title to the underlying commodity. Therefore, gains and losses from settlement of financial swaps and options are recorded net in operating revenues in the accompanying consolidated statements of operations. Revenues under certain long-term power sales contracts are recognized based on the output delivered at the lower of the amount billable or the average rate over the contract term. The excess of the amounts billed over the portion recorded as revenues is reflected in deferred revenues on the consolidated balance sheets. EME accounts for grant income on the deferred method and, accordingly, will recognize operating revenues related to such income over the estimated useful life of the projects. EME received US Treasury Grants for eligible wind projects of $44 million and $388 million in 2012 and 2011, respectively.
Power Purchase Agreements	Power Purchase Agreements (EME only) EME enters into long-term power purchase agreements in the normal course of business. A power purchase agreement may be considered a variable interest in a VIE. Under this classification, the power purchase agreement is evaluated to determine if EME is the primary beneficiary in the VIE, in which case, such entity would be consolidated. EME does not have any power purchase agreements in which it is the primary beneficiary. A power purchase agreement may also contain a lease for accounting purposes. This generally occurs when a power purchase agreement (signed or modified after June 30, 2003) designates a specific power plant in which the buyer purchases substantially all of the output and does not otherwise meet a fixed price per unit of output exception. EME has a number of power purchase agreements that contain leases in which EME is considered the lessor. These agreements are classified as operating leases. EME records rental income under these contracts as electricity is delivered at rates defined in power sales agreements. Revenues from these power sales agreements were $224 million, $124 million and $109 million in 2013, 2012 and 2011, respectively. A power purchase agreement that does not contain a lease may be classified as a derivative subject to a normal purchases and sales exception, in which case the power purchase agreement is classified as an executory contract. The contracts that are not eligible for the normal purchases and sales exception are defined as a derivative and are recorded on the consolidated balance sheets at fair value. For further information on derivatives and hedging activities, see Note 6—Derivative Instruments and Hedging Activities. Power purchase agreements that do not meet the preceding classification are accounted for on the accrual basis.
Derivative Instruments and Hedging Activities	Derivative Instruments and Hedging Activities Authoritative guidance on derivatives and hedging establishes accounting and reporting standards for derivative instruments (including certain derivative instruments embedded in other contracts). EME and Midwest Generation are required to record derivatives on their balance sheets as either assets or liabilities measured at fair value unless otherwise exempted from derivative treatment as normal purchases and sales. All changes in the fair value of derivative instruments are recognized currently in earnings, unless specific hedge criteria are met, which requires that EME and Midwest Generation formally document, designate, and assess the effectiveness of transactions that receive hedge accounting. The accounting guidance for cash flow hedges provides that the effective portion of gains or losses on derivative instruments designated and qualifying as cash flow hedges be reported as a component of other comprehensive loss and be reclassified into earnings in the same period during which the hedged forecasted transaction affects earnings. The remaining gains or losses on the derivative instruments, if any, must be recognized currently in earnings.
Stock-Based Compensation	Stock-Based Compensation (EME only) EIX's stock options, performance shares, deferred stock units and restricted stock units have been granted to EME employees under EIX's long-term incentive compensation programs. EME employees ceased receiving new awards under these programs upon the commencement of the Chapter 11 Cases, however, existing awards continue to vest. Upon completion of the NRG Sale and emergence from bankruptcy, all of EME's employees will be terminated. The terminated employees will generally receive one additional year of vesting and any unvested long-term incentive compensation awards will be canceled. Generally, EIX does not issue new common stock for settlement of equity awards. Rather, a third party is used to purchase shares from the market and deliver for settlement of option exercises, performance shares, and restricted stock units. EIX has discretion to settle certain performance shares awards in common stock; however, awards are generally settled half in cash and half in common stock. Deferred stock units granted to management are settled in cash and represent a liability. Restricted stock units are settled in common stock; however, EIX will substitute cash awards to the extent necessary to pay tax withholding or any government levies. EME recognizes stock-based compensation expense on a straight-line basis over the requisite service period. EME recognizes stock-based compensation expense for awards granted to retirement-eligible participants on a prorated basis over the initial year or over the period between the date of grant and the date the participant first becomes eligible for retirement. In conjunction with the commencement of the Chapter 11 Cases, EME ceased participating in EIX's long-term incentive compensation programs, and no new EIX stock-based compensation was awarded to EME employees in 2013.
Income Taxes and Tax-Allocation Agreements	Income Taxes and Tax-Allocation Agreements EME Historically, EME participated in tax-allocation agreements with EIX in which EME would be eligible to receive payments from EIX for tax losses and credits generated by EME. During 2012, EIX modified the tax-allocation agreements to terminate EME's participation on December 31, 2013. The parties to the Settlement Agreement have estimated the EME Tax Attributes to be $1.19 billion and have agreed that EIX will pay the Reorganization Trust amounts equal to 50% of the EME Tax Attributes as follows: $225 million payable on the Effective Date in cash, with one half of the balance payable on each of September 30, 2015 and September 30, 2016, together with interest at 5% per annum from the Effective Date. If the Settlement Agreement is terminated, as a result of the termination of the Acquisition Agreement or otherwise, then, as of January 1, 2014 benefits that had been previously generated by EME and utilized in the EIX consolidated tax return on a statutory basis but are unpaid under the tax-allocation agreements will not be available for use by EME in its own consolidated tax return and will not be paid by EIX. Additionally, tax benefits that had previously been generated by EME and not utilized in the EIX consolidated tax return on a statutory basis will generally be available for use by EME in its own consolidated return, but may be reduced by cancellation of indebtedness income (COD income) or as a result of the application of the consolidated return rules. Under the Plan, EME will continue to be included in the consolidated federal and combined state income tax returns of EIX. EME's tax provision is determined using the “benefits for losses” method. This method is similar to a separate company return, except that EME recognizes, without regard to separate company limitations, additional tax liabilities or benefits based on the impact to the combined group including EME's taxable income or losses and state apportionment factors. Realization of any tax benefits generated by EME is dependent on EME's continued inclusion in the consolidated EIX tax returns, and the generation of sufficient consolidated taxable income by the EIX consolidated tax group prior to the expiration of the loss and credit carryforwards. Differences between the amount recorded in tax provision under the benefits for losses method and the amount of cash expected to be paid or received through the intercompany tax allocation agreements is recorded to equity. EME accounts for deferred income taxes using the asset-and-liability method, wherein deferred tax assets and liabilities are recognized for future tax consequences of temporary differences between the carrying amounts and the tax bases of assets and liabilities using enacted income tax rates. Historically, in evaluating the realization of tax sharing assets, EME determined whether it was more likely than not the EIX consolidated tax group would generate sufficient taxable income to utilize EME's deferred income tax assets during the period in which EME was a part of the EIX consolidated income tax returns, or if it was more likely than not EME would utilize the deferred income tax assets on its own, after its expected separation from the consolidated group. As a result, as of December 31, 2013 and 2012, EME recorded a valuation allowance on its net deferred tax assets of $752 million and $444 million, respectively. Upon the effectiveness of the Settlement Agreement, EME would expect to reverse the valuation allowance it has recorded. For further information, see Note 7—Income Taxes. Investment and energy tax credits are deferred and amortized over the term of the power purchase agreement of the respective project while production tax credits are recognized when earned. EME's investments in wind-powered electric generation projects qualify for federal production tax credits, unless a US Treasury Grant has been elected. Certain of EME's wind projects also qualify for state tax credits, which are accounted for similarly to federal production tax credits. Interest income, interest expense and penalties associated with income taxes are reflected in provision (benefit) for income taxes on EME's consolidated statements of operations. Midwest Generation Midwest Generation is included in the consolidated federal and state income tax returns of EIX and is party to a tax-allocation agreement with its parent Edison Mission Midwest Holdings (the Midwest Generation Tax Allocation Agreement). Midwest Generation's tax allocation method is to allocate current tax liabilities or benefits on a separate return basis, except for the use of state tax apportionment factors of the EIX group for purposes of determining state income taxes. The Midwest Generation Tax Allocation Agreement only permits the use of net operating losses to offset future taxable income and does not include the right to receive payments. Accordingly, if Midwest Generation offsets net operating loss carryforwards against taxable income in the future, such tax benefits are accounted for as non-cash equity contributions from its parent at the time of use. Tax benefits recognized associated with net operating losses carrybacks that are not paid under the Midwest Generation Tax Allocation Agreement are accounted for as non-cash distributions to the parent company. Midwest Generation accounts for deferred income taxes using the asset-and-liability method, wherein deferred tax assets and liabilities are recognized for future tax consequences of temporary differences between the carrying amounts and the tax bases of assets and liabilities using enacted income tax rates. In evaluating the realization of deferred income tax assets, Midwest Generation must determine whether it is more likely than not it would utilize its own deferred income tax assets in a hypothetical tax return prepared on a separate company basis. As a result, as of December 31, 2013 and 2012, Midwest Generation recorded a valuation allowance on its net deferred tax assets of $767 million and $533 million, respectively. For further information regarding the valuation allowance, see Note 7—Income Taxes. Also, while Midwest Generation is generally subject to separate return limitations for net losses, under the Midwest Generation Tax Allocation Agreement it is permitted to transfer to Edison Mission Midwest Holdings, or its subsidiaries, net operating loss benefits or other current or deferred tax attributions, which would not yet be realized in a separate return in exchange for a reduction in Midwest Generation's intercompany account balances (including subordinated loans). Differences between amounts recorded in tax provision based on a hypothetical tax return prepared on a separate company basis and the amount of cash expected to be paid or received through the Midwest Generation Tax Allocation Agreement are recorded to equity. Under the Acquisition Agreement, prior to the closing date, Midwest Generation may reorganize into a single-member limited liability corporation, in which case it will become a disregarded entity for tax purposes and its existing income tax attributes will accumulate to EME. Under the Settlement Agreement, the EME Tax Attributes will be retained by the EIX consolidated tax group. Interest income, interest expense and penalties associated with income taxes are reflected in benefit for income taxes on Midwest Generation's consolidated statements of operations.
New Accounting Guidance	New Accounting Guidance Accounting Guidance Adopted in 2013 Offsetting Assets and Liabilities In December 2011 and December 2012, the Financial Accounting Standards Board (FASB) issued accounting standards updates modifying the disclosure requirements about the nature of an entity's rights of offsetting assets and liabilities in the consolidated balance sheet under master netting agreements and related arrangements associated with financial and derivative instruments. The guidance requires increased disclosure of the gross and net recognized assets and liabilities, collateral positions, and narrative descriptions of setoff rights. EME and Midwest Generation adopted this guidance effective January 1, 2013. Presentation of Items Reclassified out of Accumulated Other Comprehensive Income In February 2013, the FASB issued an accounting standards update which requires disclosure related to items reclassified out of accumulated other comprehensive income (AOCI). The guidance requires entities to present separately, for each component of other comprehensive income (OCI), current period reclassifications and the remainder of the current-period OCI. In addition, for certain current period reclassifications, an entity is required to disclose the effect of the item reclassified out of AOCI on the respective line item of net income. EME adopted this guidance effective January 1, 2013.