Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	$ 9	[1]	$ 19	[1]
Derivative Instruments	4		5
Net Amount	5		14
Gross Amounts Recognized in the Balance Sheet	258	[1]	232	[1]
Derivative Instruments	4		5
Cash Collateral Received/Posted	6	[2]	5	[2]
Net Amount	248		222
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	8	[1]	18	[1]
Derivative Instruments	4		5
Net Amount	4		13
Gross Amounts Recognized in the Balance Sheet	44	[1]	46	[1]
Derivative Instruments	4		5
Cash Collateral Received/Posted	4	[2]	5	[2]
Net Amount	36		36
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	1		1
Net Amount	1		1
Gross Amounts Recognized in the Balance Sheet	214	[1]	186	[1]
Cash Collateral Received/Posted	2	[2]
Net Amount	$ 212		$ 186

[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.