Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Union Electric Company [Member]
Related Party Transaction [Line Items]
Total Operating Revenues	$ 20		$ 19		$ 19
Operating Expenses	106		114		136
Union Electric Company [Member] | Ameren Missouri Power Supply Agreements with Ameren Illinois [Member]
Related Party Transaction [Line Items]
Total Operating Revenues			2		2
Union Electric Company [Member] | Ameren Missouri and Genco Gas Transportation Agreement [Member]
Related Party Transaction [Line Items]
Total Operating Revenues	1		1		1
Union Electric Company [Member] | Ameren Missouri and Ameren Illinois Rent and Facility Services [Member]
Related Party Transaction [Line Items]
Total Operating Revenues	19		16		16
Union Electric Company [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri [Member]
Related Party Transaction [Line Items]
Interest (Charges) Income
Union Electric Company [Member] | Ameren Services Support Services Agreement [Member]
Related Party Transaction [Line Items]
Operating Expenses	106		114		128
Union Electric Company [Member] | AFS Support Services Agreement [Member]
Related Party Transaction [Line Items]
Operating Expenses					7
Union Electric Company [Member] | Insurance Premiums [Member]
Related Party Transaction [Line Items]
Operating Expenses					1	[1]
Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Total Operating Revenues	16		11		11
Gas Purchased for Resale					1
Operating Expenses	88		87		102
Ameren Illinois Company [Member] | Ameren Missouri and Ameren Illinois Rent and Facility Services [Member]
Related Party Transaction [Line Items]
Total Operating Revenues	1		1		1
Ameren Illinois Company [Member] | Ameren Illinois Transmission Services Agreement with Marketing Company [Member]
Related Party Transaction [Line Items]
Total Operating Revenues	15		10		10
Ameren Illinois Company [Member] | Ameren Illinois Power Supply Agreements with Marketing Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	311		232		233
Ameren Illinois Company [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri [Member]
Related Party Transaction [Line Items]
Operating Expenses			2		2
Interest (Charges) Income
Ameren Illinois Company [Member] | Purchased Power [Member]
Related Party Transaction [Line Items]
Operating Expenses	311		234		235
Ameren Illinois Company [Member] | Ameren Services Support Services Agreement [Member]
Related Party Transaction [Line Items]
Operating Expenses	88		87		102
Maximum [Member] | Union Electric Company [Member] | Ameren Missouri Power Supply Agreements with Ameren Illinois [Member]
Related Party Transaction [Line Items]
Total Operating Revenues	1	[2]
Maximum [Member] | Union Electric Company [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri [Member]
Related Party Transaction [Line Items]
Interest (Charges) Income	1	[2]
Maximum [Member] | Union Electric Company [Member] | Insurance Premiums [Member]
Related Party Transaction [Line Items]
Other Operations and Maintenance	1	[2]	1	[2]
Maximum [Member] | Ameren Illinois Company [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri [Member]
Related Party Transaction [Line Items]
Total Purchased Power	1	[2]
Interest (Charges) Income	1	[2]
Maximum [Member] | Ameren Illinois Company [Member] | AFS Support Services Agreement [Member]
Related Party Transaction [Line Items]
Other Operations and Maintenance					$ 1	[2]

[1] Represents insurance premiums paid to Energy Risk Assurance Company, an affiliate for replacement power, property damage, and terrorism coverage.

[2] Amount less than $1 million.