Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 20	[1]	$ 31	[1]	$ 61	[1]	$ 94	[1]
Pension Plan, Defined Benefit [Member] | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	13		18		38		54
Pension Plan, Defined Benefit [Member] | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	7		10		22		31
Pension Plan, Defined Benefit [Member] | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	[2]	3	[3]	1		9	[3]
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(3)	[1]	2	[1]	(6)	[1]	7	[1]
Other Postretirement Benefit Plan, Defined Benefit [Member] | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	[2]	2		2		7
Other Postretirement Benefit Plan, Defined Benefit [Member] | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(3)		1	[2]	(7)		1	[2]
Other Postretirement Benefit Plan, Defined Benefit [Member] | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	[2]	1	[2],[3]	(1)		1	[2],[3]
New Ameren Energy Resources Company, LLC | Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost			2				8
New Ameren Energy Resources Company, LLC | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost			$ 1	[2]			$ 1	[2]

[1] Includes amounts for Ameren registrants and nonregistrant subsidiaries.

[2] Less than $1 million.

[3] Includes $2 million and $8 million in total net costs for pension benefits for the three and nine months ended September 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income. Includes less than $1 million in total net costs for postretirement benefits for both the three and nine months ended September 30, 2013, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income.