Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) - Commodity Contract - USD ($) $ in Millions		Sep. 30, 2015		Dec. 31, 2014
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		$ 24	[1]	$ 19	[2]
Derivative Instruments		2		5
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		22		14
Gross Amounts Recognized in the Balance Sheet		255	[1]	232	[2]
Derivative Instruments		2		5
Cash Collateral Received/Posted	[3]	7		5
Net Amount		246		222
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		24	[1]	18	[2]
Derivative Instruments		2		5
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		22		13
Gross Amounts Recognized in the Balance Sheet		41	[1]	46	[2]
Derivative Instruments		2		5
Cash Collateral Received/Posted	[3]	6		5
Net Amount		33		36
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		0		1
Derivative Instruments		0		0
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		0		1
Gross Amounts Recognized in the Balance Sheet		214	[1]	186	[2]
Derivative Instruments		0		0
Cash Collateral Received/Posted	[3]	1		0
Net Amount		$ 213		$ 186

[1] The derivative asset and liability balances are presented net of counterparty credit considerations.

[2] The derivative asset and liability balances are presented net of counterparty credit considerations.

[3] 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.