Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) - Commodity Contract - USD ($) $ in Millions		Jun. 30, 2015		Dec. 31, 2014
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		$ 31	[1]	$ 19	[2]
Derivative Instruments		3		5
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		28		14
Gross Amounts Recognized in the Balance Sheet		237	[1]	232	[2]
Derivative Instruments		3		5
Cash Collateral Received/Posted	[3]	2		5
Net Amount		232		222
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		30	[1]	18	[2]
Derivative Instruments		3		5
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		27		13
Gross Amounts Recognized in the Balance Sheet		34	[1]	46	[2]
Derivative Instruments		3		5
Cash Collateral Received/Posted	[3]	2		5
Net Amount		29		36
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		1		1
Derivative Instruments		0		0
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		1		1
Gross Amounts Recognized in the Balance Sheet		203	[1]	186	[2]
Derivative Instruments		0		0
Cash Collateral Received/Posted	[3]	0		0
Net Amount		$ 203		$ 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.