Summary Of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2014
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 significant 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.
Earnings Per Share	There were no material differences between Ameren’s basic and diluted earnings per share amounts for the three months ended March 31, 2014, and 2013, caused by the assumed settlement of performance share units. The number of dilutive performance share units had an immaterial impact on earnings per share.
Stock-based Compensation	Ameren’s long-term incentive plan available for eligible employees, the 2006 Omnibus Incentive Compensation Plan (2006 Plan), was replaced prospectively for new grants only by the 2014 Omnibus Incentive Compensation Plan (2014 Plan) effective April 24, 2014. The 2014 Plan provides for a maximum of 8 million common shares to be available for grant to eligible employees and directors, and retains many of the features of the 2006 Plan. The 2014 Plan awards may be stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares, performance share units, cash-based awards, and other stock-based awards.
Intangible Assets	Ameren and Ameren Missouri classify renewable energy credits and emission allowances as intangible assets. Ameren Illinois consumes renewable energy credits as they are purchased through the IPA procurement process and expenses them immediately. We evaluate intangible assets for impairment if events or changes in circumstances indicate that their carrying amount might be impaired. At March 31, 2014, Ameren’s and Ameren Missouri’s intangible assets consisted of renewable energy credits obtained through wind and solar power purchase agreements. The book value of Ameren’s and Ameren Missouri’s renewable energy credits was $17 million each at March 31, 2014. The book value of Ameren’s and Ameren Missouri’s renewable energy credits was $22 million each at December 31, 2013. Ameren Missouri’s and Ameren Illinois’ renewable energy credits and Ameren Missouri’s emission allowances are charged to “Purchased power” expense and “Fuel” expense, respectively, as they are used in operations.
Excise Taxes	Excise taxes levied on us are reflected on Ameren Missouri electric customer bills and on Ameren Missouri and Ameren Illinois natural gas customer bills. They 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 reflected on Ameren Illinois electric customer bills are imposed on the customer and are therefore not included in revenues and expenses. They are included in “Taxes accrued” on the balance sheet.
Uncertain Tax Positions	Ameren’s federal income tax returns for the years 2007 through 2011 are before the Appeals Office of the IRS. Ameren’s federal income tax return for the year 2012 is currently under examination. It is reasonably possible that a settlement will be reached with the Appeals Office of the IRS in the next 12 months for the years 2007 through 2011. This settlement, which is primarily related to uncertain tax positions for research tax deductions, is expected to result in a decrease in uncertain tax benefits of $20 million and $13 million for Ameren and Ameren Missouri, respectively. In addition, it is reasonably possible that other events will occur during the next 12 months that would cause the total amount of unrecognized tax benefits for the Ameren Companies to increase or decrease. However, the Ameren Companies do not believe any such increases or decreases, including the decrease from the reasonably possible IRS Appeals Office settlement discussed above, would be material to their results of operations, financial position, or liquidity. State income tax returns are generally subject to examination for a period of three years after filing of the return. The Ameren Companies do not currently have material state income tax issues under examination, administrative appeals, 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. With the adoption of new accounting guidance in the first quarter of 2014, unrecognized tax benefits are recorded in “Accumulated deferred income taxes, net” as a reduction to the deferred tax assets for net operating loss and tax credit carryforwards on Ameren’s, Ameren Missouri’s and Ameren Illinois’ balance sheets. Unrecognized tax benefits that exceed these carryforwards are recorded in “Other deferred credits and liabilities” on the respective balance sheets. At March 31, 2014, unrecognized tax benefits of $84 million, $14 million, and $- million were recorded in “Accumulated deferred income taxes, net” on Ameren's, Ameren Missouri's and Ameren Illinois' balance sheets, respectively. At December 31, 2013, unrecognized tax benefits of $84 million, $15 million, and $- million previously recorded in “Other deferred credits and liabilities” on the respective balance sheets were reclassified to “Accumulated deferred income taxes, net” for comparative purposes.
Asset Retirement Obligations	AROs at Ameren, Ameren Missouri and Ameren Illinois increased at March 31, 2014, compared to December 31, 2013, to reflect the accretion of obligations to their fair value, partially offset by immaterial settlements.
Noncontrolling Interest	As of March 31, 2014, Ameren's noncontrolling interests were composed of the preferred stock not subject to mandatory redemption of Ameren Missouri and Ameren Illinois. All noncontrolling interests are classified as a component of equity separate from Ameren's equity on its consolidated balance sheet.
Accounting and Reporting Developments	The following is a summary of recently adopted authoritative accounting guidance. Presentation of an Unrecognized Tax Benefit In July 2013, FASB issued additional authoritative accounting guidance to provide clarity for the financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. The objective of this guidance is to eliminate diversity in practice related to the presentation of certain unrecognized tax benefits. It requires entities to present an unrecognized tax benefit as a reduction to a deferred tax asset for a net operating loss carryforward, a similar tax loss, or a tax credit carryforward to the extent a net operating loss carryforward, a similar tax loss, or a tax credit carryforward is available under the tax law. This guidance was effective for the Ameren Companies beginning in the first quarter of 2014. Previously, unrecognized tax benefits were recorded in “Other deferred credits and liabilities” on Ameren's, Ameren Missouri's and Ameren Illinois' balance sheets. Beginning in the first quarter 2014, unrecognized tax benefits are recorded in “Accumulated deferred income taxes, net” as a reduction to the deferred tax assets for net operating loss and tax credit carryforwards on the respective balance sheets. Unrecognized tax benefits that exceed these carryforwards are recorded in “Other deferred credits and liabilities,” on the respective balance sheets. For comparative purposes, the Ameren Companies reclassified the December 31, 2013 balances in accordance with the new guidance as discussed in the Uncertain Tax Positions section above. The implementation of the additional authoritative accounting guidance did not affect the Ameren Companies' results of operations or liquidity, as this guidance is presentation-related only.

Summary Of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2014

The financial statements of Ameren are prepared on a consolidated basis, and therefore include the accounts of its majority-owned subsidiaries. All significant 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.

Earnings Per Share

There were no material differences between Ameren’s basic and diluted earnings per share amounts for the three months ended March 31, 2014, and 2013, caused by the assumed settlement of performance share units. The number of dilutive performance share units had an immaterial impact on earnings per share.

Stock-based Compensation

Ameren’s long-term incentive plan available for eligible employees, the 2006 Omnibus Incentive Compensation Plan (2006 Plan), was replaced prospectively for new grants only by the 2014 Omnibus Incentive Compensation Plan (2014 Plan) effective April 24, 2014. The 2014 Plan provides for a maximum of 8 million common shares to be available for grant to eligible employees and directors, and retains many of the features of the 2006 Plan. The 2014 Plan awards may be stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares, performance share units, cash-based awards, and other stock-based awards.

Intangible Assets

Ameren and Ameren Missouri classify renewable energy credits and emission allowances as intangible assets. Ameren Illinois consumes renewable energy credits as they are purchased through the IPA procurement process and expenses them immediately. We evaluate intangible assets for impairment if events or changes in circumstances indicate that their carrying amount might be impaired.

At March 31, 2014, Ameren’s and Ameren Missouri’s intangible assets consisted of renewable energy credits obtained through wind and solar power purchase agreements. The book value of Ameren’s and Ameren Missouri’s renewable energy credits was $17 million each at March 31, 2014. The book value of Ameren’s and Ameren Missouri’s renewable energy credits was $22 million each at December 31, 2013.

Ameren Missouri’s and Ameren Illinois’ renewable energy credits and Ameren Missouri’s emission allowances are charged to “Purchased power” expense and “Fuel” expense, respectively, as they are used in operations.

Excise Taxes

Excise taxes levied on us are reflected on Ameren Missouri electric customer bills and on Ameren Missouri and Ameren Illinois natural gas customer bills. They 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 reflected on Ameren Illinois electric customer bills are imposed on the customer and are therefore not included in revenues and expenses. They are included in “Taxes accrued” on the balance sheet.

Uncertain Tax Positions

Ameren’s federal income tax returns for the years 2007 through 2011 are before the Appeals Office of the IRS. Ameren’s federal income tax return for the year 2012 is currently under examination.

It is reasonably possible that a settlement will be reached with the Appeals Office of the IRS in the next 12 months for the years 2007 through 2011. This settlement, which is primarily related to uncertain tax positions for research tax deductions, is expected to result in a decrease in uncertain tax benefits of $20 million and $13 million for Ameren and Ameren Missouri, respectively. In addition, it is reasonably possible that other events will occur during the next 12 months that would cause the total amount of unrecognized tax benefits for the Ameren Companies to increase or decrease. However, the Ameren Companies do not believe any such increases or decreases, including the decrease from the reasonably possible IRS Appeals Office settlement discussed above, would be material to their results of operations, financial position, or liquidity.

State income tax returns are generally subject to examination for a period of three years after filing of the return. The Ameren Companies do not currently have material state income tax issues under examination, administrative appeals, 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.

With the adoption of new accounting guidance in the first quarter of 2014, unrecognized tax benefits are recorded in “Accumulated deferred income taxes, net” as a reduction to the deferred tax assets for net operating loss and tax credit carryforwards on Ameren’s, Ameren Missouri’s and Ameren Illinois’ balance sheets. Unrecognized tax benefits that exceed these carryforwards are recorded in “Other deferred credits and liabilities” on the respective balance sheets. At March 31, 2014, unrecognized tax benefits of $84 million, $14 million, and $- million were recorded in “Accumulated deferred income taxes, net” on Ameren's, Ameren Missouri's and Ameren Illinois' balance sheets, respectively. At December 31, 2013, unrecognized tax benefits of $84 million, $15 million, and $- million previously recorded in “Other deferred credits and liabilities” on the respective balance sheets were reclassified to “Accumulated deferred income taxes, net” for comparative purposes.

Asset Retirement Obligations

AROs at Ameren, Ameren Missouri and Ameren Illinois increased at March 31, 2014, compared to December 31, 2013, to reflect the accretion of obligations to their fair value, partially offset by immaterial settlements.

Noncontrolling Interest

As of March 31, 2014, Ameren's noncontrolling interests were composed of the preferred stock not subject to mandatory redemption of Ameren Missouri and Ameren Illinois. All noncontrolling interests are classified as a component of equity separate from Ameren's equity on its consolidated balance sheet.

Accounting and Reporting Developments

The following is a summary of recently adopted authoritative accounting guidance.

Presentation of an Unrecognized Tax Benefit

In July 2013, FASB issued additional authoritative accounting guidance to provide clarity for the financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. The objective of this guidance is to eliminate diversity in practice related to the presentation of certain unrecognized tax benefits. It requires entities to present an unrecognized tax benefit as a reduction to a deferred tax asset for a net operating loss carryforward, a similar tax loss, or a tax credit carryforward to the extent a net operating loss carryforward, a similar tax loss, or a tax credit carryforward is available under the tax law. This guidance was effective for the Ameren Companies beginning in the first quarter of 2014. Previously, unrecognized tax benefits were recorded in “Other deferred credits and liabilities” on Ameren's, Ameren Missouri's and Ameren Illinois' balance sheets. Beginning in the first quarter 2014, unrecognized tax benefits are recorded in “Accumulated deferred income taxes, net” as a reduction to the deferred tax assets for net operating loss and tax credit carryforwards on the respective balance sheets. Unrecognized tax benefits that exceed these carryforwards are recorded in “Other deferred credits and liabilities,” on the respective balance sheets. For comparative purposes, the Ameren Companies reclassified the December 31, 2013 balances in accordance with the new guidance as discussed in the Uncertain Tax Positions section above. The implementation of the additional authoritative accounting guidance did not affect the Ameren Companies' results of operations or liquidity, as this guidance is presentation-related only.