Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) - Commodity Contract - USD ($) $ in Millions		Sep. 30, 2016		Dec. 31, 2015
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		$ 15	[1]	$ 18	[2]
Derivative Instruments		4		1
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		11		17
Gross Amounts Recognized in the Balance Sheet		215	[1]	264	[2]
Derivative Instruments		4		1
Cash Collateral Received/Posted	[3]	5		11
Net Amount		206		252
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		12	[1]	17	[2]
Derivative Instruments		2		1
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		10		16
Gross Amounts Recognized in the Balance Sheet		25	[1]	44	[2]
Derivative Instruments		2		1
Cash Collateral Received/Posted	[3]	5		8
Net Amount		18		35
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		3		1
Derivative Instruments		2		0
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		1		1
Gross Amounts Recognized in the Balance Sheet		190	[1]	220	[2]
Derivative Instruments		2		0
Cash Collateral Received/Posted	[3]	0		3
Net Amount		$ 188		$ 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.