Related Party Transactions (Schedule of Related Party Transactions) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Union Electric Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	$ 6		$ 5
Operating Expenses	31		28
Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	6		2
Operating Expenses	25		23
Ameren Missouri Power Supply Agreements with Ameren Illinois [Member] | Union Electric Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	1		1	[1]
Ameren Missouri and Ameren Illinois Rent and Facility Services [Member] | Union Electric Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	5		5
Ameren Missouri and Ameren Illinois Rent and Facility Services [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	1	[1]	1	[1]
Ameren Missouri and Genco Gas Transportation Agreement [Member] | Union Electric Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	1	[1]	1	[1]
Ameren Illinois Transmission Services Agreement with Marketing Company [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	6		2
Ameren Illinois Power Supply Agreements with Marketing Company [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	26		87
Purchased Power [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	27		87
Ameren Services Support Services Agreement [Member] | Union Electric Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	31		28
Ameren Services Support Services Agreement [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	25		23
Ameren Illinois Power Supply Agreements with Ameren Missouri [Member] | Union Electric Company [Member]
Related Party Transaction [Line Items]
Interest Charges	1	[1]	0
Ameren Illinois Power Supply Agreements with Ameren Missouri [Member] | Ameren Illinois Company [Member]
Related Party Transaction [Line Items]
Interest Charges	1	[1]	1	[1]
Operating Expenses	1		1	[1]
Insurance Premiums [Member] | Union Electric Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	$ 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, property damage and terrorism coverage.