Impairment and Other Charges (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended											1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended		3 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2012 Union Electric Company [Member]		Dec. 31, 2011 Union Electric Company [Member]		Dec. 31, 2010 Union Electric Company [Member]	Dec. 31, 2011 Closure of Meredosia and Hutsonville Energy Centers [Member]	Dec. 31, 2011 Taum Sauk Energy Center [Member] Union Electric Company [Member]	Jul. 31, 2011 SO2 Emission Allowances [Member]	Dec. 31, 2010 SO2 Emission Allowances [Member]	Jul. 31, 2011 SO2 Emission Allowances [Member] Union Electric Company [Member]	Dec. 31, 2012 Merchant Generation [Member]	Dec. 31, 2010 Merchant Generation [Member]	Mar. 31, 2012 Merchant Generation [Member] Duck Creek Energy Center [Member]
Impairment and Other Charges [Line Items]
Impairment charge on long-lived assets and related charges	$ 2,578	[1]	$ 123	[1]	$ 101	[1]			$ 89			$ 26					$ 1,950	$ 101	$ 628
Property, Plant and Equipment, Net	16,096	[2],[3]	18,127	[2],[3]			10,161	[3]	9,958	[3]							748
Non-cash impairment of materials and supplies												4
Severance costs												4
Loss from regulatory disallowance									89				89
Impairment charge on goodwill	0	[1]	0	[1]	420	[1]			0									420
Pretax impairment charge	$ 0	[1]	$ 2	[1]	$ 68	[1]			$ 0					$ 2	$ 68	$ 1

[1] Includes amounts for registrant and nonregistrant subsidiaries.

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

[3] 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 $228 million and $229 million at December 31, 2012, and 2011, respectively. The total accumulated depreciation associated with the two CTs was $52 million and $52 million at December 31, 2012, and 2011, respectively. In addition, Ameren Missouri has investments in debt securities, which are classified as held-to-maturity, related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2012, and 2011, the carrying value of these debt securities was $304 million and $309 million, respectively.