Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Restricted Cash and Cash Equivalents		$ 8
Aggregate Fair Value of Derivative Liabilities	[1]	97
Cash Collateral Posted		3
Potential Aggregate Amount of Additional Collateral Required	[2]	80
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	[1]	64
Cash Collateral Posted		3
Potential Aggregate Amount of Additional Collateral Required	[2]	54
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	[1]	33
Cash Collateral Posted		0
Potential Aggregate Amount of Additional Collateral Required	[2]	26
Cash | Union Electric Company
Derivative [Line Items]
Restricted Cash and Cash Equivalents		$ 12	$ 11
Cash | Ameren Illinois Company
Derivative [Line Items]
Restricted Cash and Cash Equivalents			$ 3

[1] Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures.

[2] As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.