Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	$ 41	[1]	$ 29	[1]
Derivative Instruments	11		10
Cash Collateral Received/Posted		[2]		[2]
Net Amount	30		19
Gross Amounts Recognized in the Balance Sheet	182	[1]	230	[1]
Derivative Instruments	11		10
Cash Collateral Received/Posted	32	[2]	65	[2]
Net Amount	139		155
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	41	[1]	28	[1]
Derivative Instruments	11		9
Cash Collateral Received/Posted		[2]		[2]
Net Amount	30		19
Gross Amounts Recognized in the Balance Sheet	26	[1]	25	[1]
Derivative Instruments	11		9
Cash Collateral Received/Posted	8	[2]	7	[2]
Net Amount	7		9
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet			1
Derivative Instruments			1
Cash Collateral Received/Posted		[2]		[2]
Net Amount
Gross Amounts Recognized in the Balance Sheet	156	[1]	205	[1]
Derivative Instruments			1
Cash Collateral Received/Posted	24	[2]	58	[2]
Net Amount	$ 132		$ 146

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

[2] Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet.