Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended										1 Months Ended			12 Months Ended								3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2012 Ameren Illinois Company [Member] sqmi people	Dec. 31, 2012 Union Electric Company [Member] sqmi people	Dec. 31, 2011 Union Electric Company [Member]	Dec. 31, 2012 Electric Energy Inc [Member]	Feb. 29, 2012 Medina Valley Energy Center [Member]	Jun. 30, 2011 Columbia CT Energy Center [Member]	Jun. 30, 2010 Columbia CT Energy Center [Member]	Dec. 31, 2011 Shutdown Of Meredosia And Hutsonville Energy Centers [Member]	Dec. 31, 2012 Merchant Generation Separation Program [Member]	Dec. 31, 2010 Merchant Generation Separation Program [Member]	Dec. 31, 2011 Other Asset Sales [Member]	Dec. 31, 2012 Other Asset Sales [Member] Merchant Generation [Member]	Dec. 31, 2012 Voluntary Separation Offer [Member]	Dec. 31, 2011 SO2 Emission Allowances [Member]	Dec. 31, 2011 SO2 Emission Allowances [Member] Union Electric Company [Member]	Dec. 31, 2009 Minimum [Member]	Dec. 31, 2012 Minimum [Member]	Dec. 31, 2011 Minimum [Member]	Dec. 31, 2009 Maximum [Member]	Dec. 31, 2012 Maximum [Member]	Dec. 31, 2011 Maximum [Member]	Dec. 31, 2012 Maximum [Member] Union Electric Company [Member] Emission Allowances [Member]	Dec. 31, 2012 Power [Member] Ameren Illinois Company [Member] customer	Dec. 31, 2012 Power [Member] Union Electric Company [Member] customer	Dec. 31, 2012 Natural Gas [Member] Ameren Illinois Company [Member] customer	Dec. 31, 2012 Natural Gas [Member] Union Electric Company [Member] customer	Dec. 31, 2012 FAC [Member] Union Electric Company [Member]
Accounting Policies [Line Items]
Tax grants received related to renewable energy properties	$ 18							$ 18
Public Utilities, Area Serviced							40,000	24,000
Public Utilities, Estimated Population of Service Territory							3,100,000	2,800,000
Public Utilities, Number of Customers																													1,200,000	1,200,000	806,000	127,000
Ownership percentage by parent										80.00%
Percent of average depreciable cost																						3.00%	3.00%	3.00%	4.00%	4.00%	4.00%
Book value	16		7					14	7
Amortization of regulatory asset																												1
Pretax impairment charge	0	[1]	2	[1]	68	[1]			0											2	1
Sharing Level For Fac																																	95.00%
Percentage of EEI not owned by Ameren										20.00%
Number of employee positions eliminated																			340
Severance costs														4	1	4			28
Proceeds from sales of properties	22		53		27				27		16	45	18				4	5
Additional contingent proceeds from sale of properties											1
Pretax gain recognized on sale											10	8	5				4	1
Percent of property sold													25.00%
Impairment charge on goodwill	$ 0	[1]	$ 0	[1]	$ 420	[1]			$ 0

[1] Includes amounts for registrant and nonregistrant subsidiaries.