Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 20	$ 23	$ 60	$ 69
Interest cost		46	43	138	130
Expected return on plan assets		(63)	(62)	(189)	(186)
Prior service cost (benefit)		0	0	0	0
Actuarial loss		8	19	24	56
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		0	0	0	1
Net periodic benefit cost	[1],[2]	11	23	33	70
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		5	6	15	17
Interest cost		12	12	36	36
Expected return on plan assets		(18)	(17)	(54)	(51)
Prior service cost (benefit)		(1)	(1)	(3)	(3)
Actuarial loss		(3)	1	(8)	4
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		0	0	0	0
Net periodic benefit cost	[1],[2]	(5)	1	(14)	3
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		6	14	19	42
Union Electric Company | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		(1)	2	(3)	6
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		6	9	17	28
Ameren Illinois Company | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		(3)		(10)	(2)
Other Affiliated Entities and Intercompany Eliminations [Member] | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		(1)	0	(3)	0
Other Affiliated Entities and Intercompany Eliminations [Member] | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		$ (1)	$ (1)	$ (1)	$ (1)

[1] (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

[2] (b)Includes amounts for Ameren registrants and nonregistrant subsidiaries.