Related Party Transactions (Schedule of Related Party Transactions) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	$ 5		$ 6
Operating Expenses	33		31
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	[1]	1	[1]
Operating Expenses	27		25
Purchased Power | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	1	[2]	1	[2]
Purchased Power | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1		2
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	[1]	1
Ameren Missouri Power Supply Agreements with Ameren Illinois | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	[2]	1	[2]
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	5		5
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	[1]	1	[1]
Ameren Illinois Power Supply Agreements with Ameren Missouri | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	1	[2]	1	[2]
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	[1]	1
Ameren Illinois Transmission Agreements with ATXI [Member] | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	1	[2]	1	[2]
Ameren Illinois Transmission Agreements with ATXI [Member] | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1		1
Ameren Services Support Services Agreement | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	33		31
Ameren Services Support Services Agreement | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	27		25
Insurance Premiums | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	1	[1],[3]	1	[1],[3]
Insurance Premiums | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	[2],[3]	1	[2],[3]
Related Party Money Pool Interest | Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	1	[1]	1	[1]
Related Party Money Pool Interest | Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	$ 1	[1]	$ 1	[1]

[1] Amount less than $1 million.

[2] Not applicable.

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