Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	$ 16	[1]	$ 34	[1]
Derivative Instruments	8		10
Net Amount	8		24
Gross Amounts Recognized in the Balance Sheet	167	[1]	178	[1]
Derivative Instruments	8		10
Cash Collateral Received/Posted	5	[2]	24	[2]
Net Amount	154		144
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	14	[1]	33	[1]
Derivative Instruments	7		9
Net Amount	7		24
Gross Amounts Recognized in the Balance Sheet	21	[1]	24	[1]
Derivative Instruments	7		9
Cash Collateral Received/Posted	5	[2]	9	[2]
Net Amount	9		6
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	2		1
Derivative Instruments	1		1
Net Amount	1		0
Gross Amounts Recognized in the Balance Sheet	146	[1]	154	[1]
Derivative Instruments	1		1
Cash Collateral Received/Posted			15	[2]
Net Amount	$ 145		$ 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.