Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	$ 34	[1]	$ 34	[1]
Derivative Instruments	12		10
Net Amount	22		24
Gross Amounts Recognized in the Balance Sheet	149	[1]	178	[1]
Derivative Instruments	12		10
Cash Collateral Received/Posted	10	[2]	24	[2]
Net Amount	127		144
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	30	[1]	33	[1]
Derivative Instruments	9		9
Net Amount	21		24
Gross Amounts Recognized in the Balance Sheet	22	[1]	24	[1]
Derivative Instruments	9		9
Cash Collateral Received/Posted	10	[2]	9	[2]
Net Amount	3		6
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	4		1
Derivative Instruments	3		1
Net Amount	1		0
Gross Amounts Recognized in the Balance Sheet	127	[1]	154	[1]
Derivative Instruments	3		1
Cash Collateral Received/Posted			15	[2]
Net Amount	$ 124		$ 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.