Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) - Commodity Contract - USD ($) $ in Millions		Mar. 31, 2016		Dec. 31, 2015
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		$ 7	[1]	$ 18	[2]
Derivative Instruments		3		1
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		4		17
Gross Amounts Recognized in the Balance Sheet		281	[1]	264	[2]
Derivative Instruments		3		1
Cash Collateral Received/Posted	[3]	10		11
Net Amount		268		252
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		6		17	[2]
Derivative Instruments		2		1
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		4		16
Gross Amounts Recognized in the Balance Sheet		43	[1]	44	[2]
Derivative Instruments		2		1
Cash Collateral Received/Posted	[3]	8		8
Net Amount		33		35
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		1		1
Derivative Instruments		1		0
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		0		1
Gross Amounts Recognized in the Balance Sheet		238	[1]	220	[2]
Derivative Instruments		1		0
Cash Collateral Received/Posted	[3]	2		3
Net Amount		$ 235		$ 217

[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.