Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	$ 28	[1]	$ 34	[1]
Derivative Instruments	12		10
Cash Collateral Received/Posted		[2]		[2]
Net Amount	16		24
Gross Amounts Recognized in the Balance Sheet	172	[1]	178	[1]
Derivative Instruments	12		10
Cash Collateral Received/Posted	12	[2]	24	[2]
Net Amount	148		144
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	23	[1]	33	[1]
Derivative Instruments	9		9
Cash Collateral Received/Posted		[2]		[2]
Net Amount	14		24
Gross Amounts Recognized in the Balance Sheet	23	[1]	24	[1]
Derivative Instruments	9		9
Cash Collateral Received/Posted	9	[2]	9	[2]
Net Amount	5		6
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	5		1
Derivative Instruments	3		1
Cash Collateral Received/Posted		[2]		[2]
Net Amount	2
Gross Amounts Recognized in the Balance Sheet	149	[1]	154	[1]
Derivative Instruments	3		1
Cash Collateral Received/Posted	3	[2]	15	[2]
Net Amount	$ 143		$ 138

[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.