Rate And Regulatory Matters (Narrative) (Details) (USD $)	12 Months Ended										1 Months Ended			60 Months Ended		0 Months Ended		0 Months Ended	1 Months Ended	12 Months Ended					12 Months Ended		1 Months Ended		12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009	Jun. 30, 2010 FERC Relicensing [Member] Taum Sauk Energy Center [Member]	Dec. 31, 2011 Approved By FERC And MISO [Member] Potential Transmission Project Investments 2012 Through 2016 [Member]	Dec. 31, 2011 Approved By FERC And MISO [Member] Potential Transmission Project Investments Through 2019 [Member]	Oct. 28, 2011 MoPSC Staff Recommendation [Member] FAC Prudence Review [Member] Subsequent Periods After September 30, 2009 [Member]	Dec. 31, 2011 Ameren Missouri [Member] Renewable Energy Portfolio Requirement [Member]	Apr. 30, 2011 Ameren Missouri [Member] FAC Prudence Review [Member]	Sep. 30, 2011 Ameren Missouri [Member] FAC Prudence Review [Member] months	Sep. 30, 2011 Ameren Missouri [Member] FAC Prudence Review [Member] Subsequent Periods After September 30, 2009 [Member]	Dec. 31, 2005 Ameren Missouri [Member] Pending FERC Case [Member] Power Purchase Agreement With Entergy Arkansas [Member]	Dec. 31, 2011 Ameren Missouri [Member] 2009 Final Rate Order [Member] Electric Distribution [Member]	Jul. 13, 2011 Ameren Missouri [Member] Final Rate Order [Member] Electric Distribution [Member]	May 31, 2010 Ameren Missouri [Member] Final Rate Order [Member] Electric Distribution [Member]	Feb. 03, 2012 Ameren Missouri [Member] Pending Rate Order [Member] Electric Distribution [Member]	Jan. 20, 2012 Ameren Missouri [Member] Pending Rate Order [Member] MEEIA Filing [Member] Electric Distribution [Member]	Dec. 31, 2011 Ameren Illinois Company [Member]	Dec. 31, 2010 Ameren Illinois Company [Member]	Dec. 31, 2009 Ameren Illinois Company [Member]		Jan. 31, 2011 Ameren Illinois Company [Member] Wholesale Distribution Rate Case [Member]	Dec. 31, 2011 Ameren Illinois Company [Member] IEIMA [Member] Requirements [Member]	Dec. 31, 2010 Ameren Illinois Company [Member] Final Rate Order [Member] Electric And Gas Distribution [Member]	Jan. 31, 2012 Ameren Illinois Company [Member] Final Rate Order [Member] Gas Distribution [Member]	Jan. 03, 2012 Ameren Illinois Company [Member] Initial Filing [Member] IEIMA [Member] Electric Distribution [Member]	Dec. 31, 2011 Callaway Unit 2 [Member]
Rate And Regulatory Matters [Line Items]
Authorized increase in revenue from utility service																$ 173,000,000	$ 230,000,000									$ 40,000,000	$ 32,000,000
Amount held by Circuit Court based on appeal of electric rate order															20,000,000		15,000,000
Number of industrial customers who received a stay from Circuit Court																	4
Increase in normalized net fuel costs																52,000,000		103,000,000
Utility revenue increase requested																		376,000,000	81,000,000					11,000,000
Rate of return on common equity																10.20%		10.75%									9.06%
Percent of capital structure composed of equity																52.20%		52.00%									53.30%
Rate base																6,600,000,000		6,800,000,000									1,000,000,000
Recovery and refund period																12 months to eight months
Energy Efficiency program spending																			145,000,000
Number of years energy efficiency spending will occur																			3
Sharing level for FAC																95.00%
Request to defer fixed costs not recovered from Noranda, amount												36,000,000
Time required to complete FAC prudence reviews, in months												18
Percentage of electric native load sales required to be purchased or generated from renewable energy sources minimum										2.00%
Percentage of electric native load sales required to be purchased or generated from renewable energy sources by 2021										15.00%
Percentage limitation on customer rate increases attributed to renewable energy source requirements										1.00%
Percentage of each portfolio requirement that must be derived from solar energy										2.00%
Authorized decrease in revenue from utility service																												19,000,000
Excess basis points over treasury year one																									590
Excess basis points over treasury after year one																									580
Collar basis points																									50
The first three years' maximum basis point penalty																									30
The middle four years' maximum basis point penalty																									34
The last four years' maximum basis point penalty																									38
Maximum residential rate increase allowed on an annual basis under IEIMA																									2.50%
Incremental capital expenditure requirement																									625,000,000
New jobs created requirement																									450
Annual contribution for customer assistance programs																									1,000,000
One-Time contribution to a science and energy innovation trust																									7,500,000
Annual contribution to a science and energy innovation trust																									1,000,000
Purchased power	966,000,000		1,106,000,000		909,000,000									25,000,000						853,000,000	965,000,000	1,048,000,000	[1]
Current regulatory liabilities	133,000,000	[2]	99,000,000						26,000,000		18,000,000									76,000,000	76,000,000
Interest charges	451,000,000		497,000,000		508,000,000						1,000,000									136,000,000	143,000,000	153,000,000	[1]
Pretax earnings recognized associated with sales contracts	215,000,000	[2]	267,000,000										25,000,000							306,000,000	260,000,000
Capital investments							750,000,000	1,200,000,000
Number of years an ESP is valid for																													20
Capitalized costs relating to construction of new nuclear unit	18,127,000,000	[3],[4]	17,853,000,000	[3],[4]																4,770,000,000	4,576,000,000								69,000,000
Number of years for proposed relicensing application filed with FERC						40
Disallowed capitalized costs associated with rebuilt Taum Sauk energy center																$ 89,000,000

[1] Prior period has been adjusted to reflect the AIC Merger as discussed in Note 1 - Summary of Significant Accounting Policies.

[2] Includes intercompany eliminations.

[3] Includes amounts for Ameren registrant and nonregistrant subsidiaries as well as intercompany eliminations.

[4] Amounts in Ameren and Ameren Missouri include two electric generation CTs under two separate capital lease agreements. The gross asset value of those agreements was $229 million and $228 million at December 31, 2011 and 2010, respectively. The total accumulated depreciation associated with the two CTs was $52 million and $46 million at December 31, 2011 and 2010, respectively.