Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	$ 25		$ 20		$ 23
Operating Expenses	116		106		114
Union Electric Company | Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	3		1	[1]	2
Union Electric Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	21		19		16
Union Electric Company | Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues	1		1		5
Union Electric Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Interest (Charges) Income
Union Electric Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	116		106		114
Union Electric Company | Insurance Premiums
Related Party Transaction [Line Items]
Operating Expenses	1	[1],[2]	1	[1],[2]	1	[1],[2]
Union Electric Company | Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	1	[1]	1	[1]
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	4		1		3
Operating Expenses	93		88		87
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	1		1		2
Ameren Illinois Company | Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues	3		1	[1]	1
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	3		1	[1]	2
Interest (Charges) Income
Ameren Illinois Company | Ameren Illinois transmission agreements with ATXI
Related Party Transaction [Line Items]
Operating Expenses	2		3		3
Ameren Illinois Company | Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	5		3		5
Ameren Illinois Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	93		88		87
Ameren Illinois Company | Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	$ 1	[1]	$ 1	[1]

[1] Amount less than $1 million.

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