Summary Of Significant Accounting Policies (Policies)	3 Months Ended
Summary Of Significant Accounting Policies (Policies)	Mar. 31, 2015
Accounting Policies [Abstract]
Consolidation	The financial statements of Ameren are prepared on a consolidated basis, and therefore include the accounts of its majority-owned subsidiaries. All intercompany transactions have been eliminated. Ameren Missouri and Ameren Illinois have no subsidiaries, and therefore their financial statements are not prepared on a consolidated basis. All tabular dollar amounts are in millions, unless otherwise indicated. Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair presentation of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and the notes thereto included in the Form 10-K.
Excise Taxes	Excise taxes are levied on Ameren Missouri’s electric and natural gas businesses and on Ameren Illinois’ natural gas business and are recorded gross in “Operating Revenues - Electric,” “Operating Revenues - Gas” and “Operating Expenses - Taxes other than income taxes” on the statement of income or the statement of income and comprehensive income. Excise taxes for electric service in Illinois are levied on the customer and are therefore not included in Ameren Illinois’ revenues and expenses.
Uncertain Tax Positions	Ameren’s federal income tax return for the tax year 2013 is currently under examination by the IRS. It is reasonably possible that a settlement will be reached with the IRS in the next 12 months, which will result in a reduction of Ameren’s unrecognized tax benefits of $53 million related to discontinued operations. In addition, it is reasonably possible that other events will occur during the next 12 months that would cause the total amount of our unrecognized tax benefits to fluctuate. However, other than as described above, we do not believe any such fluctuations would be material to our results of operations, financial position, or liquidity. State income tax returns are generally subject to examination for a period of three years after filing. We do not currently have material state income tax issues under examination, administrative appeal, or litigation. The state impact of any federal changes remains subject to examination by various states for a period of up to one year after formal notification to the states.
Accounting and Reporting Developments	Below is a summary of recently adopted or issued authoritative accounting guidance relevant to the Ameren Companies. Presentation of Debt Issuance Costs In April 2015, FASB issued authoritative accounting guidance to simplify the presentation of debt issuance costs. The guidance requires debt issuance costs to be presented in the balance sheet as a reduction to the associated debt liability. Currently, debt issuance costs are presented as a component of “Other assets” on the Ameren Companies’ balance sheets. The guidance will be effective for the Ameren Companies in the first quarter of 2016 and applied retrospectively. The guidance will not affect the Ameren Companies' results of operations, financial position, or liquidity, as this guidance is presentation-related only.

Disclosure of accounting policy regarding (1) the principles it follows in consolidating or combining the separate financial statements, including the principles followed in determining the inclusion or exclusion of subsidiaries or other entities in the consolidated or combined financial statements and (2) its treatment of interests (for example, common stock, a partnership interest or other means of exerting influence) in other entities, for example consolidation or use of the equity or cost methods of accounting. The accounting policy may also address the accounting treatment for intercompany accounts and transactions, noncontrolling interest, and the income statement treatment in consolidation for issuances of stock by a subsidiary.

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