Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[1]	$ 92	$ 81	$ 109	[2]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[1]	4	(8)	1	[2]
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[3]	54	50	69
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[3]	8	3	8
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		38	30	41
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(3)	(9)	0
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		0	1	5
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(1)	(2)	0
Parent Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[4]	92	81	115
Parent Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[4]	$ 4	$ (8)	$ 8

[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] (a)Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.

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