Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	$ 37	[1]	$ 29	[1]
Derivative Instruments	20		10
Cash Collateral Received/Posted		[2]		[2]
Net Amount	17		19
Gross Amounts Recognized in the Balance Sheet	183	[1]	230	[1]
Derivative Instruments	20		10
Cash Collateral Received/Posted	39	[2]	65	[2]
Net Amount	124		155
Union Electric Company [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	28	[1]	28	[1]
Derivative Instruments	15		9
Cash Collateral Received/Posted		[2]		[2]
Net Amount	13		19
Gross Amounts Recognized in the Balance Sheet	32	[1]	25	[1]
Derivative Instruments	15		9
Cash Collateral Received/Posted	8	[2]	7	[2]
Net Amount	9		9
Ameren Illinois Company [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	9		1
Derivative Instruments	5		1
Cash Collateral Received/Posted		[2]		[2]
Net Amount	4
Gross Amounts Recognized in the Balance Sheet	151	[1]	205	[1]
Derivative Instruments	5		1
Cash Collateral Received/Posted	31	[2]	58	[2]
Net Amount	$ 115		$ 146

[1] The derivative asset and liability balances are presented net of counterparty credit considerations.

[2] Cash collateral received reduces gross asset balances and cash collateral posted reduces gross liability balances.