Long-Term Debt And Financings (Schedule Of Coverage Ratios) (Detail) (USD $)	6 Months Ended										12 Months Ended
	Jun. 30, 2012 Ameren Missouri [Member]		Jun. 30, 2012 Ameren Missouri [Member] Actual [Member] Bond Restriction [Member]	Jun. 30, 2012 Ameren Missouri [Member] Minimum Required Ratio [Member] Bond Restriction [Member]		Jun. 30, 2012 Ameren Illinois Company [Member]		Jun. 30, 2012 Ameren Illinois Company [Member] Actual [Member] Bond Restriction [Member]	Jun. 30, 2012 Ameren Illinois Company [Member] Minimum Required Ratio [Member] Bond Restriction [Member]		Jun. 30, 2012 Genco [Member] Restricted Payments [Member] Minimum [Member]		Jun. 30, 2012 Genco [Member] Additional Indebtedness [Member] Maximum [Member]		Jun. 30, 2012 Genco [Member] Actual [Member]	Jun. 30, 2012 Genco [Member] Maximum Leverage Ratio [Member]
Long-Term Debt And Equity Financings [Line Items]
Actual nterest overage Ratio			3.7	2.0	[1]			7.1	2.0	[1]	1.75	[2]	2.5	[3]	3.39
Bonds Issuable	$ 2,780	[4]				$ 3,514	[4],[5]
Dividend Coverage Ratio			100.3	2.5	[6]			3.0	1.5	[6]
Preferred Stock Issuable	1,912					203
Retired bond capacity	$ 89,000,000					$ 765,000,000
Actual Debt-to- Capital Ratio															0.43	0.60	[3]

[1] Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds.

[2] A minimum interest coverage ratio of 1.75 is required for Genco to make certain restricted payments, as defined, including specified dividend payments and, principal and interest payments on subordinated borrowings. As of the date of the restricted payment, the minimum ratio must have been achieved for the most recently ended four fiscal quarters and projected by management to be achieved for each of the subsequent four six-month periods. Investments in the non-state-regulated subsidiary money pool and repayments of non-state-regulated subsidiary money pool borrowings are not subject to this incurrence test.

[3] A minimum interest coverage ratio of 2.50 for the most recently ended four fiscal quarters and a debt-to-capital ratio of no greater than 60% are required for Genco to incur additional indebtedness, as defined, other than permitted indebtedness, as defined, for borrowed money. The ratios must be computed on a pro forma basis considering the additional indebtedness to be incurred and the related interest expense. Non-state-regulated subsidiary money pool borrowings are defined as permitted indebtedness and are not subject to these incurrence tests. Credit facility borrowings, including borrowings under the 2010 Genco Credit Agreement, and other borrowings from third-party, external sources are included in the definition of indebtedness and are subject to these incurrence tests.

[4] Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $89 million and $765 million at Ameren Missouri and Ameren Illinois, respectively.

[5] Amount of bonds issuable by Ameren Illinois based on unfunded property additions and retired bonds solely under the former IP mortgage indenture.

[6] Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company's articles of incorporation.