Related Party Transactions (Schedule of Related Party Transactions) (Detail) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	$ 8,000,000		$ 5,000,000		$ 21,000,000		$ 18,000,000
Operating Expenses	25,000,000		25,000,000		90,000,000		85,000,000
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1,000,000	[1]	1,000,000	[1]	1,000,000		3,000,000
Operating Expenses	26,000,000		22,000,000		80,000,000		70,000,000
Purchased Power | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]
Purchased Power | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	3,000,000		1,000,000		7,000,000		3,000,000
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	2,000,000		1,000,000	[1]	5,000,000		1,000,000
Ameren Missouri Power Supply Agreements with Ameren Illinois | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	6,000,000		4,000,000		15,000,000		16,000,000
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1,000,000	[1]	1,000,000	[1]	1,000,000		1,000,000
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1,000,000	[1]	1,000,000		1,000,000		1,000,000
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]	2,000,000
Ameren Illinois Power Supply Agreements with Ameren Missouri | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	2,000,000		1,000,000	[1]	5,000,000		1,000,000
Ameren Illinois Transmission Agreements with ATXI | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1,000,000		1,000,000		2,000,000		2,000,000
Ameren Services Support Services Agreement | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	25,000,000		25,000,000		90,000,000		85,000,000
Ameren Services Support Services Agreement | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	26,000,000		22,000,000		80,000,000		70,000,000
Insurance Premiums | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	1,000,000	[1],[3]	1,000,000	[1],[3]	1,000,000	[1],[3]	1,000,000	[1],[3]
Insurance Premiums | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1,000,000	[2],[3]	1,000,000	[2],[3]	1,000,000	[2],[3]	1,000,000	[2],[3]
Related Party Money Pool Interest | Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]
Related Party Money Pool Interest | Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	$ 1,000,000	[1]	$ 1,000,000	[1]	$ 1,000,000	[1]	$ 1,000,000	[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.