Retirement Benefits (Components Of Net Periodic Benefit Cost) (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]
Service cost	$ 79	[1],[2]	$ 91	[1],[2]	$ 81	[1]
Interest cost	183	[1],[2]	163	[1],[2]	166	[1]
Expected return on plan assets	(229)	[1]	(218)	[1]	(208)	[1]
Prior service credit	(1)	[1]	(2)	[1]	(3)	[1]
Actuarial (gain) loss	49	[1]	87	[1]	75	[1]
Curtailment			(12)	[1]
Net periodic benefit cost (benefit)	81	[1]	109	[1],[3]	111	[1],[4]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	19	[1],[2]	22	[1],[2]	22	[1]
Interest cost	50	[1],[2]	46	[1],[2]	47	[1]
Expected return on plan assets	(65)	[1]	(62)	[1]	(56)	[1]
Transition obligation					2	[1]
Prior service credit	(5)	[1]	(6)	[1]	(6)	[1]
Actuarial (gain) loss	(7)	[1]	8	[1]	5	[1]
Curtailment			(7)	[1]
Net periodic benefit cost (benefit)	(8)	[1]	1	[1],[3]	14	[1],[4]
New Ameren Energy Resources Company, LLC | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (benefit)	6	[1],[3]	9	[1],[3]
New Ameren Energy Resources Company, LLC | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (benefit)	$ 7	[1],[3]

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries

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

[3] (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.

[4] (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.