Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract [Member], USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	$ 57	[1]	$ 29	[1]
Derivative Instruments	15		10
Cash Collateral Received/Posted		[2]		[2]
Net Amount	42		19
Gross Amounts Recognized in the Balance Sheet	183	[1]	230	[1]
Derivative Instruments	15		10
Cash Collateral Received/Posted	32	[2]	65	[2]
Net Amount	136		155
Ameren Missouri [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	53	[1]	28	[1]
Derivative Instruments	13		9
Cash Collateral Received/Posted		[2]		[2]
Net Amount	40		19
Gross Amounts Recognized in the Balance Sheet	28	[1]	25	[1]
Derivative Instruments	13		9
Cash Collateral Received/Posted	6	[2]	7	[2]
Net Amount	9		9
Ameren Illinois Company [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	4		1
Derivative Instruments	2		1
Cash Collateral Received/Posted		[2]		[2]
Net Amount	2
Gross Amounts Recognized in the Balance Sheet	155	[1]	205	[1]
Derivative Instruments	2		1
Cash Collateral Received/Posted	26	[2]	58	[2]
Net Amount	$ 127		$ 146

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

[2] Cash collateral received reduces gross asset balances and cash collateral posted reduces gross liability balances.