Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Rate And Regulatory Matters [Line Items]
Current regulatory assets	$ 215	[1]	$ 267
Noncurrent regulatory assets	1,603	[1]	1,263
Current regulatory liabilities	133	[1]	99
Noncurrent regulatory liabilities	1,502	[1]	1,319
Under-Recovered FAC [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory assets	83	[1],[2],[3]	158	[2],[3]
Under-Recovered Illinois Electric Power Costs [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory assets	4	[1],[2],[4]	4	[2],[4]
Under-Recovered PGA [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory assets	8	[1],[2],[4]	2	[2],[4]
MTM Derivative Losses [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory assets	120	[1],[5]	103	[5]
Noncurrent regulatory assets	100	[1],[5]	85	[5]
Pension And Postretirement Benefit Costs [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	878	[1],[6]	555	[6]
Income Taxes [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	239	[1],[7]	230	[7]
Noncurrent regulatory liabilities	48	[1],[8]	54	[8]
Asset Retirement Obligation [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	6	[1],[9]	9	[9]
Noncurrent regulatory liabilities	29	[1],[9]
Callaway Costs [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	48	[1],[10],[2]	51	[10],[2]
Unamortized Loss On Reacquired Debt [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	47	[1],[11],[2]	53	[11],[2]
Recoverable Costs Contaminated Facilities [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	102	[1],[12]	127	[12]
SO2 Emission Allowances Sale Tracker [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	6	[1],[13]	12	[13]
Storm Costs [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	16	[1],[14]	23	[14]
Demand-Side Costs [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	70	[1],[15]	39	[15]
Reserve For Workers' Compensation Liabilities [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	13	[1],[16]	14	[16]
Bad Debt Rider [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	10	[1],[13]	5	[13]
Credit Facilities Fees [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	10	[1],[17]	12	[17]
Employee Separation Costs [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	6	[1],[18]	8	[18]
Common Stock Issuance Costs [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	10	[1],[19]	12	[19]
Construction Accounting For Pollution Control Equipment [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	25	[1],[2],[20]	4	[2],[20]
Other [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory assets	27	[1],[21]	29	[21]
Noncurrent regulatory liabilities	2	[1],[22]	5	[22]
Over-Recovered FAC [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	12	[1],[23]
Over-Recovered Illinois Electric Power Costs [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	66	[1],[4]	62	[4]
Over-Recovered PGA [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	9	[1],[4]	12	[4]
MTM Derivative Gains [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	46	[1],[24]	25	[24]
Noncurrent regulatory liabilities	82	[1],[24]	20	[24]
Removal Costs [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	1,269	[1],[25]	1,177	[25]
Pension And Postretirement Benefit Costs Tracker [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	38	[1],[26]	45	[26]
Energy Efficiency Rider [Member]
Rate And Regulatory Matters [Line Items]
Noncurrent regulatory liabilities	$ 24	[1],[27]	$ 13	[27]

[1] Includes intercompany eliminations.

[2] These assets earn a return.

[3] Under-recovered fuel costs for periods from July 2009 through December 2011. Specific accumulation periods aggregate the under-recovered costs over four months, any related adjustments occur over the following four months, and then recovery from customers occurs over the next eight months.

[4] Costs under- or over-recovered from utility customers. Amounts will be recovered from, or refunded to, customers within one year of the deferral.

[5] Deferral of commodity-related derivative MTM losses, as well as the MTM losses on financial contracts entered into by Ameren Illinois with Marketing Company.

[6] These costs are being amortized in proportion to the recognition of prior service costs (credits), transition obligations (assets), and actuarial losses (gains) attributable to Ameren's pension plan and postretirement benefit plans. See Note 11 - Retirement Benefits for additional information.

[7] Offset to certain deferred tax liabilities for expected recovery of future income taxes when paid. See Note 13 - Income Taxes for amortization period.

[8] Unamortized portion of investment tax credit and federal excess deferred taxes. See Note 13 - Income Taxes for amortization period.

[9] Recoverable or refundable removal costs for AROs at our rate-regulated operations, including net realized and unrealized gains and losses related to the nuclear decommissioning trust fund investments. See Note 1 - Summary of Significant Accounting Policies - Asset Retirement Obligations.

[10] Ameren Missouri's Callaway energy center operations and maintenance expenses, property taxes, and carrying costs incurred between the plant in-service date and the date the plant was reflected in rates. These costs are being amortized over the remaining life of the plant's current operating license (through 2024).

[11] Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the remaining lives of the old debt issuances if no new debt was issued.

[12] The recoverable portion of accrued environmental site liabilities, primarily collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of actual expenditures. See Note 15 - Commitments and Contingencies for additional information.

[13] A regulatory tracking mechanism for the difference between the level of bad debt expense incurred by Ameren Illinois under GAAP and the level of such costs included in electric and natural gas rates. The over-recovery relating to 2010 is being refunded to customers from June 2011 through May 2012. The over-recovery relating to 2011 will be refunded to customers from June 2012 through May 2013.

[14] Actual storm costs in a test year that exceed the MoPSC staff's normalized storm costs for rate purposes. The 2006 storm costs are being amortized until July 2013. The 2008 storm costs are being amortized over five years, beginning on March 1, 2009. In addition, the balance includes January 2007 ice storm costs that Ameren Missouri will recover over five years, beginning in March 2009, as approved by the January 2009 MoPSC electric rate order. The 2009 storm costs are being amortized over five years, beginning in July 2010, as approved by the May 2010 MoPSC electric rate order.

[15] Demand-side costs, including the costs of developing, implementing and evaluating customer energy efficiency and demand response programs. Costs incurred from May 2008 through September 2008 are being amortized over 10 years, beginning in March 2009. Costs incurred from October 2008 through December 2009 are being amortized over six years, beginning in July 2010. Costs incurred from January 2010 through February 2011 are being amortized over six years, beginning in August 2011. The amortization period for the costs incurred after February 2011 will be determined in Ameren Missouri's pending electric rate case.

[16] Reserve for workers' compensation claims.

[17] Ameren Missouri's costs incurred to enter into and maintain the 2009 multiyear and supplemental credit agreements, prior to their termination in 2010. These costs are being amortized over two years, beginning in July 2010, as approved by the May 2010 MoPSC electric rate order. These costs are being amortized to construction work in progress, which will be subsequently depreciated when assets are placed into service.

[18] Cost incurred for the voluntary and involuntary separation programs. The 2009 Ameren Missouri-related costs are being amortized over three years, beginning in July 2010, as approved by the May 2010 MoPSC electric rate order. The 2009 Ameren Illinois-related costs are being amortized over three years, beginning in May 2010, as approved by the April 2010 ICC electric and natural gas rate order.

[19] The MoPSC's May 2010 electric rate order allowed Ameren Missouri to recover its portion of Ameren's September 2009 common stock issuance costs. These costs are being amortized over five years, beginning in July 2010.

[20] The MoPSC's May 2010 electric rate order allowed Ameren Missouri to continue recording an allowance for funds used during construction for pollution control equipment at its Sioux energy center until the cost of that equipment is placed in customer rates. The amortization of these costs will be over the expected life of the Sioux energy center.

[21] Includes costs related to Ameren Illinois' delivery service rate cases that resulted in orders in 2008 and 2010 as well as the natural gas delivery service rate case that resulted in an order in January 2012. The natural gas costs associated with the 2008 rate case will be amortized until September 2013. The 2010 rate case costs are being amortized over a two-year period, beginning in May 2010. The 2012 natural gas rate case costs will be amortized over a two year period, beginning in January 2012. The Ameren Illinois total also includes a portion of the unamortized debt fair value adjustment recorded upon Ameren's acquisition of IP. This portion is being amortized over the remaining life of the related debt, beginning with the expiration of the electric rate freeze in Illinois on January 1, 2007. The Ameren Illinois total also includes Ameren Illinois Merger integration and optimization costs. These costs will be amortized over four years, beginning in January 2012. At Ameren Missouri, the balance includes cost associated with the retirement of renewable energy credits and solar rebates to fulfill Ameren Missouri's renewable energy portfolio requirement. The amortization period for these costs will be determined in Ameren Missouri's pending electric rate case. The Ameren Missouri balance also includes a regulatory tracking mechanism for the difference between the level of vegetation management and infrastructure inspection costs incurred by Ameren Missouri under GAAP and the level of such costs included in electric rates. Ameren Missouri's vegetation management and infrastructure inspection costs from July 2011 through December 2011 were more than the amount allowed in base rates. The amortization period for these costs will be determined in Ameren Missouri's pending electric rate case.

[22] Balance includes a regulatory tracking mechanism for the difference between the level of vegetation management and infrastructure inspection costs incurred by Ameren Missouri under GAAP and the level of such costs included in electric rates. Ameren Missouri's vegetation management and infrastructure inspection costs from July 2010 through February 2011 were less than the amount allowed in base rates. The over-recovery incurred during that time period is being amortized over three years beginning in August 2011. The balance also includes the deferral of gains on emission allowance vintage swaps Ameren Missouri entered into during 2005. The balance of this gain was immaterial at the end of 2011.

[23] Over-recovered fuel costs from March 2009 through September 2009 as ordered by the MoPSC in April 2011. Customer refunds will conclude in May 2012.

[24] Deferral of commodity-related derivative MTM gains.

[25] Estimated funds collected for the eventual dismantling and removal of plant from service, net of salvage value, upon retirement related to our rate-regulated operations. See discussion in Note 1 - Summary of Significant Accounting Policies - Asset Retirement Obligations.

[26] A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs built into electric rates. The 2008 costs are being amortized through February 2014. The 2009 costs are being amortized through June 2015. The 2010 costs assigned to the natural gas and electric businesses are being amortized through February 2016 and July 2016, respectively. The 2011 costs will be determined in Ameren Missouri's pending electric rate case.

[27] A regulatory tracking mechanism that allows Ameren Illinois to recover its electric and natural gas costs associated with developing, implementing and evaluating customer energy efficiency and demand response programs. This over-recovery will be refunded to customers over the following 12 months after the plan year.