Derivative Financial Instruments Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	$ 34	[1]	$ 29	[1]
Derivative Instruments	10		10
Cash Collateral Received/Posted		[2]		[2]
Net Amount	24		19
Gross Amounts Recognized in the Balance Sheet	178	[1]	230	[1]
Derivative Instruments	10		10
Cash Collateral Received/Posted	24	[2]	65	[2]
Net Amount	144		155
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	33	[1]	28	[1]
Derivative Instruments	9		9
Cash Collateral Received/Posted		[2]		[2]
Net Amount	24		19
Gross Amounts Recognized in the Balance Sheet	24	[1]	25	[1]
Derivative Instruments	9		9
Cash Collateral Received/Posted	9	[2]	7	[2]
Net Amount	6		9
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	1		1
Derivative Instruments	1		1
Cash Collateral Received/Posted		[2]		[2]
Net Amount	0		0
Gross Amounts Recognized in the Balance Sheet	154	[1]	205	[1]
Derivative Instruments	1		1
Cash Collateral Received/Posted	15	[2]	58	[2]
Net Amount	$ 138		$ 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.