Related Party Transactions (Schedule of Related Party Transactions) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	$ 7		$ 5
Operating Expenses	34		33
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1		1
Operating Expenses	29		27
Purchased Power | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	2		1
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1		1	[1]
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	6		5
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1		1	[1]
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	[1]	1	[1]
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	[1]	1	[1]
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 | 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	34		33
Ameren Services Support Services Agreement | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	29		27
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.