Rate and Regulatory Matters (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended				0 Months Ended			6 Months Ended	3 Months Ended			6 Months Ended				1 Months Ended					1 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Sep. 30, 2011	Dec. 31, 2010	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]	Jun. 30, 2011 Ameren Missouri [Member] FAC Prudence Review [Member] months	Jun. 30, 2011 Ameren Illinois [Member]	Jun. 30, 2010 Ameren Illinois [Member]		Jun. 30, 2011 Ameren Illinois [Member]	Jun. 30, 2010 Ameren Illinois [Member]		Dec. 31, 2010 Ameren Illinois [Member]	Jul. 31, 2011 Ameren Illinois [Member] Original Rate Order [Member] Gas Distribution [Member]	Feb. 28, 2011 Ameren Illinois [Member] Original Rate Order [Member] Electric Distribution [Member]	Jul. 31, 2011 Ameren Illinois [Member] Pending Rate Order [Member] Gas Distribution [Member]	Jul. 31, 2011 Ameren Illinois [Member] Pending Rate Order [Member] Electric Distribution [Member]	Jun. 30, 2011 Callaway Unit 2 [Member]	Jun. 30, 2011 Pending Rate Order [Member] ICC Staff Position [Member] Gas Distribution [Member]	Jun. 30, 2011 Pending Rate Order [Member] ICC Staff Position [Member] Electric Distribution [Member]
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								8,000,000	16,000,000
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	40,000,000		16,000,000	(10,000,000)
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											957,000,000	2,000,000,000				942,000,000	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	104,000,000	115,000,000	223,000,000	247,000,000						1,000,000	35,000,000	34,000,000	[1]	70,000,000	71,000,000	[1]
Pretax earnings recognized associated with sales contracts	184,000,000		184,000,000			267,000,000				25,000,000	243,000,000			243,000,000			260,000,000
Capitalized costs relating to construction of new nuclear unit	17,945,000,000		17,945,000,000			17,853,000,000					4,657,000,000			4,657,000,000			4,576,000,000					67,000,000
Disallowed capitalized costs associated with rebuilt Taum Sauk energy center					$ 89,000,000		$ 89,000,000

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