Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	$ 27		$ 25		$ 20
Operating Expenses	124		116		106
Union Electric Company | Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	5		3		1	[1]
Union Electric Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	21		21		19
Union Electric Company | Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues	1		1		1
Union Electric Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	124		116		106
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]	1	[1]
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	2		4		1
Operating Expenses	109		93		88
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	2		1		1
Ameren Illinois Company | Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues	1		3		1	[1]
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	5		1	[1]
Ameren Illinois Company | Ameren Illinois transmission agreements with ATXI
Related Party Transaction [Line Items]
Operating Expenses	2		2		3
Ameren Illinois Company | Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	7		5		3
Ameren Illinois Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	109		93		88
Ameren Illinois Company | Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	$ 1	[1]	$ 1	[1]	$ 0	[1]

[1] Amount less than $1 million.

[2] Represents insurance premiums paid to Missouri Energy Risk Assurance Company LLC, an affiliate, for replacement power.