Summary Of Significant Accounting Policies (Policies)	3 Months Ended
Summary Of Significant Accounting Policies (Policies)	Mar. 31, 2017
Accounting Policies [Abstract]
Revenue Recognition, Policy [Policy Text Block]	The Ameren Companies record operating revenue for electric or natural gas service when it is delivered to customers. We accrue an estimate of electric and natural gas revenues for service rendered but unbilled at the end of each accounting period. For certain regulatory recovery mechanisms qualifying as alternative revenue programs, such as revenue requirement reconciliations, the Ameren Companies recognize revenues that have been authorized for rate recovery, are objectively determinable and probable of recovery, and are expected to be collected from customers within two years.
Consolidation	Ameren’s financial statements 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. All tabular dollar amounts are in millions, unless otherwise indicated. Also see the Glossary of Terms and Abbreviations at the front of this report and in the Form 10-K. 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 statement 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. See Note 2 – Rate and Regulatory Matters for information regarding the 2017 change in Ameren Illinois' method used to recognize interim period revenue in connection with the revenue decoupling provisions of the FEJA. 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 therefore are not included in Ameren Illinois’ revenues and expenses.
Accounting and Reporting Developments	is a summary of updates related to our adoption of recently issued authoritative accounting standards. See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for additional information about recently issued authoritative accounting standards relating to leases, financial instruments, restricted cash, and the consolidation analysis for variable interest entities and voting interest entities. Revenue from Contracts with Customers In May 2014, the FASB issued authoritative guidance that changes the criteria for recognizing revenue from a contract with a customer. The underlying principle of the guidance is that an entity will recognize revenue for the transfer of promised goods or services to customers at an amount that the entity expects to be entitled to in exchange for those goods or services. The guidance requires additional disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers, as well as separate presentation of alternative revenue programs. Entities can apply the guidance to each reporting period presented, the full retrospective method, or by recording a cumulative effect adjustment to retained earnings in the period of initial adoption, the modified retrospective method. During the first quarter of 2017, we determined that contributions in aid of construction and similar arrangements are not within the scope of this guidance. Therefore, our accounting for such arrangements will not change upon adoption of this guidance. In addition, we determined that the calculation of revenue and bad debt expense for tariff sales contracts will not materially change as a result of the collectibility criterion included within the guidance. We plan to complete our assessment of the impacts of this guidance on our results of operations, financial position, disclosures, and determine our transition method, in the next few months prior to our adoption in the first quarter of 2018. Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In February 2017, the FASB issued authoritative guidance that requires an entity to retrospectively report the service cost component of net benefit cost in the same line item(s) as other compensation costs arising from services rendered by employees during the period and to present the other components of net benefit cost in the income statement separately from the service cost component, and outside of operating income. The guidance also requires that an entity only capitalize the service cost component as part of an asset such as inventory or property, plant, and equipment on a prospective basis. Previously, all of the net benefit cost components were eligible for capitalization. The adoption of this guidance in the first quarter of 2018 may result in the recognition of new regulatory assets related to the recovery of, and return on, the non-service cost components of net benefit cost and related allowance for funds used during construction balances. We are currently assessing the impacts of this guidance on our results of operations, financial position, and disclosures.