Related Party Transactions (Schedule of Related Party Transactions) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	$ 1.0				$ 1.0		$ 1.0
Operating Expenses	25		26		85		81
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues					2.0
Operating Expenses	22		22		70		67
Related Party One | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	4.0		5.0		17.0		15.0
Related Party One | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	11.0		5.0		25.0		12.0
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1.0	[1]	1.0	[1]	1.0		1.0	[1]
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	4.0		5.0		15.0		14.0
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1.0	[1]	1.0	[1]	1.0		1.0
Ameren Missouri and Genco Gas Transportation Agreement | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1.0	[1]	1.0	[1]	1.0		1.0
Ameren Illinois Transmission Services Agreement with Marketing Company | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	11.0		5.0		24.0		11.0
Ameren Illinois Power Supply Agreements with Marketing Company | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	46		83		94		243
Purchased Power | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	46		83		95		243
Ameren Services Support Services Agreement | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	25		26		85		81
Ameren Services Support Services Agreement | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	22		22		70		67
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	[1]	1	[1]	1		1	[1]
Related Party Money Pool Interest | Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	1	[1]	1	[1]	1		1	[1]
Related Party Money Pool Interest | Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	1	[1]	1	[1]	1	[1]	1	[1]
Insurance Premiums | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	$ 1	[1],[2]	$ 1	[1],[2]	$ 1	[1],[2]	$ 1	[1],[2]

[1] Amount less than $1 million.

[2] Represents insurance premiums paid to Missouri Energy Risk Assurance Company, an affiliate, for replacement power.