Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 116	[1]	$ 80	[1]	$ 65	[1]
Curtailment loss	2	[1],[2]
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	19	[1]	25	[1]	21	[1]
Curtailment loss	0	[1],[2]
Union Electric Company [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	63	[3]	51	[3]	42	[3]
Union Electric Company [Member] | Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	10	[3]	11	[3]	11	[3]
Ameren Illinois Company [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	37		16		10
Ameren Illinois Company [Member] | Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	4		11		7
Other [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	16		13		13
Other [Member] | Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	5		3		3
Electric Energy Inc [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Curtailment loss	$ 2	[3]

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries

[2] Includes EEI's pension and management postretirement benefit plans' curtailment loss of $2 million recognized in 2012 as a result of its employee reduction program.

[3] Includes amounts for Ameren registrant and nonregistrant subsidiaries.