Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2015
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation Talen Energy Corporation's obligation to report under the Securities and Exchange Act of 1934, as amended, commenced on May 1, 2015, the date Talen Energy Corporation's Registration Statement on Form S-1 relating to the spinoff transaction was declared effective by the SEC. Talen Energy Supply is a separate registrant and is considered the predecessor of Talen Energy Corporation. Therefore, the financial information prior to June 1, 2015 presented in this Quarterly Report on Form 10-Q for both registrants includes only legacy Talen Energy Supply information. From June 1, 2015, upon completion of the spinoff and acquisition, Talen Energy Corporation's and Talen Energy Supply's consolidated financial information also includes RJS. As such, Talen Energy Corporation's and Talen Energy Supply's consolidated financial information presented in this Quarterly Report on Form 10-Q for the 2015 periods represents three and nine months of legacy Talen Energy Supply information consolidated with three and four months of RJS information, while the 2014 periods represent only legacy Talen Energy Supply information. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with GAAP for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X and, therefore, do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, all adjustments considered necessary for a fair presentation in accordance with GAAP are reflected in the condensed consolidated financial statements. All adjustments are of a normal recurring nature, except as otherwise disclosed. The Balance Sheets at December 31, 2014 are derived from Talen Energy Supply's 2014 audited Balance Sheet. The financial statements and notes thereto should be read in conjunction with the financial statements and notes contained in the 2015 Prospectus. The results of operations for the three and nine months ended September 30, 2015 are not necessarily indicative of the results to be expected for the full year ending December 31, 2015 or other future periods, because results for interim periods can be disproportionately influenced by various factors, developments and seasonal variations. The classification of certain prior period amounts has been changed to conform to the presentation in the September 30, 2015 financial statements. "Income (Loss) from Discontinued Operations (net of income taxes)" for the three and nine months ended September 30, 2015 on the Statement of Income represents the activities of Sapphire, the assets and liabilities of which, at September 30, 2015, have been classified as "Assets of discontinued operations" and "Liabilities of discontinued operations" on the Balance Sheet, except for Sapphire's heat rate call options and working capital items, which may be retained, and therefore were not included in "Liabilities of discontinued operations" on the Balance Sheet. The Sapphire discontinued operations, discussed herein, excludes the heat rate call options. "Income (Loss) from Discontinued Operations (net of income taxes)" for the three and nine months ended September 30, 2014 on the Statement of Income represents the activities of Talen Montana's hydroelectric generating facilities sold in the fourth quarter of 2014. The Statements of Cash Flows do not separately report the cash flows of discontinued operations. See Note 7 for additional information.
New Accounting Guidance	Reporting of Discontinued Operations Effective January 1, 2015, Talen Energy prospectively adopted accounting guidance that changes the criteria for determining what should be classified as a discontinued operation and the related presentation and disclosure requirements. A discontinued operation may include a component of an entity or a group of components of an entity, or a business activity. A disposal of a component of an entity or a group of components of an entity is required to be reported in discontinued operations if the disposal represents a strategic shift that has (or will have) a major effect on the entity's operations and financial results when any of the following occurs: (1) the components of an entity or a group of components of an entity meets the criteria to be classified as held for sale, (2) the component of an entity or a group of components of an entity is disposed of by sale, or (3) the component of an entity or a group of components of an entity is disposed of other than by sale (for example, by abandonment or in a distribution to owners in a spinoff). The initial adoption of this guidance did not have a significant impact on Talen Energy but will impact the amounts presented as discontinued operations and will enhance the related disclosure requirements related to future disposals or held for sale classifications. Accounting for Measurement-Period Adjustments Effective September 30, 2015, Talen Energy prospectively adopted accounting guidance that requires an acquirer in a business combination to recognize measurement-period adjustments in the period in which the amounts are determined, including the effect on earnings of any amounts that would have been recorded in prior periods as if the accounting would have been completed at the acquisition date. The acquirer must disclose, by line item, the portion of the adjustment recorded in the current period income statement that would have been recognized in prior periods if the adjustment had been recognized as of the acquisition date. This guidance applies to open measurement periods, and therefore applies to any measurement period adjustments made for the acquisition of RJS Power. See Note 7 for additional information. Accounting for Revenue from Contracts with Customers In May 2014, the FASB issued accounting guidance that establishes a comprehensive new model for the recognition of revenue from contracts with customers. This model is based on the core principle that revenue should be recognized to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. This guidance can be applied using either a full retrospective or modified retrospective transition method. In August 2015, the FASB issued guidance that defers the effective date of the standard by one year, which for public business entities, results in initial application of this guidance in annual reporting periods beginning after December 15, 2017 and interim periods within those years. Entities may early adopt the guidance as of the original effective date of the standard, which for public business entities is annual reporting periods beginning after December 15, 2016. Talen Energy expects to adopt this guidance effective January 1, 2018. Talen Energy is currently assessing the impact of adopting this guidance, as well as the transition method it will use. Reporting Uncertainties about an Entity's Ability to Continue as a Going Concern In August 2014, the FASB issued accounting guidance which will require management to assess, for each interim and annual period, whether there are conditions or events that raise substantial doubt about an entity's ability to continue as a going concern. Substantial doubt about an entity's ability to continue as a going concern exists when relevant conditions and events, considered in the aggregate, indicate that it is probable that the entity will be unable to meet its obligations as they become due within one year after the date the financial statements are issued. When management identifies conditions or events that raise substantial doubt about an entity's ability to continue as a going concern, management is required to disclose information that enables users of the financial statements to understand the principal conditions or events that raised substantial doubt about the entity's ability to continue as a going concern and management's evaluation of the significance of those conditions or events. If substantial doubt about the entity's ability to continue as a going concern has been alleviated as a result of management's plan, the entity should disclose information that allows the users of the financial statements to understand those plans. If the substantial doubt about the entity's ability to continue as a going concern is not alleviated by management's plans, management's plans to mitigate the conditions or events that gave rise to the substantial doubt about the entity's ability to continue as a going concern should be disclosed, as well as a statement that there is substantial doubt the entity's ability to continue as a going concern within one year after the date the financial statements are issued. For all entities, this guidance should be applied prospectively within the annual periods ending after December 15, 2016, and for annual periods and interim periods thereafter. Early adoption is permitted. Talen Energy will adopt this guidance for the annual period ending December 31, 2016. The adoption of this guidance is not expected to have a significant impact. 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 In November 2014, the FASB issued guidance that clarifies how current accounting guidance should be interpreted when evaluating the economic characteristics and risks of a host contract of a hybrid financial instrument issued in the form of a share. This guidance does not change the current criteria for determining whether separation of an embedded derivative feature from a hybrid financial instrument is required. Entities are still required to evaluate whether the economic risks of the embedded derivative feature are clearly and closely related to those of the host contract, among other relevant criteria. An entity should consider the substantive terms and features of the entire hybrid financial instrument, including the embedded derivative feature being evaluated for bifurcation, in evaluating the nature of the host contract to determine whether the host contract is more akin to a debt instrument or more akin to an equity instrument. An entity should assess the relative strength of the debt-like and equity-like terms and features when determining how to weight those terms and features. For public business entities, this guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2015 and should be applied using a modified retrospective method for existing hybrid financial instruments issued in the form of a share as of the beginning of the fiscal year the guidance is adopted. Early adoption is permitted. Retrospective application is permitted but not required. Talen Energy will adopt this guidance on January 1, 2016. The adoption of this guidance is not expected to have a significant impact. Simplifying the Presentation of Debt Issuance Costs In April 2015, the FASB issued accounting guidance to simplify the presentation of debt issuance costs by requiring debt issuance costs to be presented on the balance sheet as a direct deduction from the carrying amount of the associated debt liability, consistent with the presentation of debt discounts. Because this guidance did not address the treatment of debt issuance costs related to line-of-credit arrangements, additional guidance was issued in August 2015 stating that an entity may defer and amortize debt issuance costs over the term of a line-of-credit arrangement, regardless of whether there are any related outstanding borrowings. For public business entities, this guidance should be applied retrospectively for financial statements issued for fiscal years beginning after December 15, 2015 and interim periods within those fiscal years. Early adoption is permitted. Talen Energy will adopt this guidance on January 1, 2016. The adoption of this guidance will require Talen Energy to reclassify debt issuance costs from assets to long-term debt, and is not expected to have a significant impact.
Interest expense allocated to discontinued operations	Represents allocated interest expense based upon the discontinued operations share of the net assets of Talen Energy.
Fair Value Transfer	Transfers between levels are recognized at end-of-reporting-period values.
Master Netting Arrangements	Net derivative positions on the balance sheets are not offset against the right to reclaim cash collateral (a receivable) or the obligation to return cash collateral (a payable) under master netting arrangements.
Impairment of goodwill and long-lived assets	U.S. GAAP requires that a long-lived asset (or asset group) be tested for recoverability whenever events or changes in circumstances indicate that the carrying amount may not be recoverable. Similarly, a goodwill impairment test is performed annually or more frequently if events or changes in circumstances indicate that more likely than not the carrying amount of a reporting unit may be greater than its fair value. During the second quarter of 2015, due to the impairment of its investment in PPL Energy Supply recorded by PPL (Talen Energy's former parent) at the time of the spinoff, coupled with, and, primarily driven by, Talen Energy Corporation's stock price at the spinoff date, Talen Energy's management concluded that these factors could be indicators of potential impairment with respect to certain long-lived assets and goodwill. After considering additional information, Talen Energy determined that the undiscounted cash flows for potentially affected long-lived assets would not be directly impacted by these factors and therefore concluded that the undiscounted cash flows continued to exceed the carrying value and no further testing of long-lived assets was necessary in the second quarter. Management also performed an interim goodwill impairment assessment as of June 1, 2015, the spinoff and acquisition date. The goodwill impairment analysis is a two-step process. The first step, used to identify potential impairment, is a comparison of the reporting unit's estimated fair value to its carrying value, including goodwill. If the fair value of the reporting unit exceeds its carrying value, applicable goodwill is not considered to be impaired. If the carrying value exceeds the fair value, there is an indication of impairment and the second step is performed to measure the amount of the impairment, if any. The second step requires a company to calculate an implied fair value of goodwill based on a hypothetical purchase price allocation. The East reporting unit, which is equivalent to the East segment continuing operations, failed step one as of June 1, 2015. The step two analysis was not able to be completed by the filing of the second quarter Form 10-Q. As provided for in the applicable accounting guidance, no goodwill impairment charge was recorded based on management's best estimate at that time, which was confirmed when the second quarter analysis was subsequently completed.
Investments	Securities held by Talen Energy's NDT funds and auction rate securities are classified as available-for-sale. Available-for-sale securities are carried on the Balance Sheets at fair value. Unrealized gains and losses on these securities are reported, net of tax, in OCI or are recognized currently in earnings when a decline in fair value is determined to be other-than-temporary. The specific identification method is used to calculate realized gains and losses.

Disclosure of accounting policy for investments in financial assets, including marketable securities (debt and equity securities with readily determinable fair values), investments accounted for under the equity method and cost method, securities borrowed and loaned, and repurchase and resale agreements. For marketable securities, the disclosure may include the entity's accounting treatment for transfers between investment categories and how the fair values for such securities are determined. Also, for all investments, an entity may describe its policy for assessing, recognizing and measuring impairment of the investment.