Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 81	[1]	$ 109	[1],[2]	$ 111	[1],[3]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(8)	[1]	1	[1],[2]	14	[1],[3]
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	50		69		63
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	3		8		10
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	30		41		37
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(9)		0		4
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	1		5		2
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(2)				0
Parent Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	81	[4]	115	[4]	102	[4]
Parent Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ (8)	[4]	$ 8	[4]	$ 14	[4]

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries

[2] (b)The net periodic benefit cost includes a $6 million and a $7 million net gain for pension benefits and postretirement benefits, respectively, which was included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). This net gain includes the curtailment gain recognized in 2013 as a result of a significant reduction in employees as of the December 2, 2013 closing date of the New AER divestiture. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.

[3] (c)The net periodic benefit cost includes $9 million and $- million in total net costs for pension benefits and postretirement benefits, respectively, which were included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.

[4] (a)Includes amounts for Ameren registrant and nonregistrant subsidiaries