Rate And Regulatory Matters (Narrative) (Details) (USD $)	3 Months Ended				9 Months Ended					9 Months Ended				0 Months Ended			3 Months Ended			9 Months Ended				1 Months Ended					9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010		Dec. 31, 2010	Sep. 30, 2011 One-Time Donation [Member] Science And Energy Innovation Trust [Member] Energy Infrastructure Modernization Act [Member]	Sep. 30, 2011 Annual Donation [Member] Science And Energy Innovation Trust [Member] Energy Infrastructure Modernization Act [Member]	Sep. 30, 2011 Annual Donation [Member] Customer Assistance Programs [Member] Energy Infrastructure Modernization Act [Member]	Sep. 30, 2011 Ameren Missouri [Member] FAC Prudence Review [Member] months	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]	Jan. 31, 2009 Ameren Missouri [Member] Final Rate Order [Member] Electric Distribution [Member]	Sep. 30, 2011 Ameren Illinois Company [Member]	Sep. 30, 2010 Ameren Illinois Company [Member]		Sep. 30, 2011 Ameren Illinois Company [Member]	Sep. 30, 2010 Ameren Illinois Company [Member]		Dec. 31, 2010 Ameren Illinois Company [Member]	Feb. 28, 2011 Ameren Illinois Company [Member] Pending Rate Order [Member] Gas Distribution [Member]	Feb. 28, 2011 Ameren Illinois Company [Member] Pending Rate Order [Member] Electric Distribution [Member]	Oct. 31, 2011 Ameren Illinois Company [Member] Additional Capital Expenditures [Member] Energy Infrastructure Modernization Act [Member]	Sep. 30, 2011 Callaway Unit 2 [Member]	Oct. 28, 2011 MoPSC Staff Recommendation [Member] FAC Prudence Review [Member]	Sep. 30, 2011 Pending Rate Order [Member] ICC Staff Position [Member] Gas Distribution [Member]	Sep. 30, 2011 Pending Rate Order [Member] ICC Staff Position [Member] Electric Distribution [Member]	Sep. 30, 2011 Approved By FERC [Member] Potential Transmission Projects Investment [Member]
Rate And Regulatory Matters [Line Items]
Authorized increase in revenue from utility service														$ 173,000,000	$ 230,000,000	$ 162,000,000
Amount held by Circuit Court based on appeal of electric rate order															14,000,000	18,000,000
Number of industrial customers who received a stay from Circuit Court															4
Increase in normalized net fuel costs														52,000,000	119,000,000
Utility revenue increase requested																								50,000,000	39,000,000				29,000,000	4,000,000
Rate of return on common equity														10.20%										10.75%	11.00%				8.90%	9.72%
Percent of capital structure composed of equity														52.20%										52.90%	52.90%				51.80%	51.80%
Rate base														6,600,000,000										956,000,000	2,000,000,000				945,000,000	2,000,000,000
Recovery and refund period														12 months to 8 months
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
Impairment of regulatory asset													18,000,000
Interest charges	113,000,000		130,000,000		336,000,000		377,000,000						1,000,000				33,000,000	37,000,000	[1]	103,000,000	108,000,000	[1]
Pretax earnings recognized associated with sales contracts	184,000,000				184,000,000				267,000,000				25,000,000				247,000,000			247,000,000			260,000,000					26,000,000
Non recoverable donation to trusts	1,000,000	[2]	7,000,000	[2]	4,000,000	[2]	10,000,000	[2]		7,500,000	1,000,000	1,000,000
Capital investments																										625,000,000					1,000,000,000
Capitalized costs relating to construction of new nuclear unit	17,873,000,000				17,873,000,000				17,853,000,000								4,699,000,000			4,699,000,000			4,576,000,000				68,000,000
Disallowed capitalized costs associated with rebuilt Taum Sauk energy center														$ 89,000,000

[1] Prior period reflects the Ameren Illinois Merger as discussed in Note 1 - Summary of Significant Accounting Policies.

[2] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.