Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment				4
External revenues		$ 1,723	$ 1,859	$ 4,775	$ 4,720
Intersegment revenues		0	0	0	0
Net Income Attributable to Ameren Common Shareholders		288	369	583	621
Capital expenditures		525	496	$ 1,523	1,496
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment				1
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment				3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues		575	676	$ 1,854	1,895
Intersegment revenues		0	0	0
Net Income Available to Common Shareholder		55	119	191	223
Capital expenditures		276	241	760	683
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues		0	0	0	0
Intersegment revenues		(26)	(31)	(78)	(80)
Net Income Available to Common Shareholder		0	0	0	0
Capital expenditures		(7)	0	(10)	0
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues		0	0	0
Intersegment revenues		(14)	(14)	(32)	(35)
Net Income Available to Common Shareholder		0	0	0
Capital expenditures		0	0	0
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues		1,104	1,150	2,799	2,733
Intersegment revenues		11	15	41	40
Net Income Available to Common Shareholder		234	241	359	347
Capital expenditures		178	147	533	500
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues		405	502	1,176	1,210
Intersegment revenues		0	1	3	3
Net Income Available to Common Shareholder		31	93	94	122
Capital expenditures		112	123	354	359
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues		111	113	509	529
Intersegment revenues		1	1	1	1
Net Income Available to Common Shareholder		2	2	40	44
Capital expenditures		71	50	180	130
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues		105	94	293	247
Intersegment revenues	[1]	14	14	33	36
Net Income Available to Common Shareholder	[2]	38	39	106	98
Capital expenditures		173	175	463	503
Operating Segments | Other
Segment Reporting Information [Line Items]
External revenues		(2)	0	(2)	1
Intersegment revenues		0	0	0	0
Net Income Available to Common Shareholder		(17)	(6)	(16)	10
Capital expenditures		(2)	1	3	4
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues		405	503	1,179	1,213
Intersegment revenues		0	0	0
Net Income Available to Common Shareholder		31	93	94	122
Capital expenditures		112	123	354	359
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues		112	114	510	530
Intersegment revenues		0	0	0
Net Income Available to Common Shareholder		2	2	40	44
Capital expenditures		71	50	180	130
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues		58	59	165	152
Intersegment revenues	[3]	14	14	32	35
Net Income Available to Common Shareholder		22	24	57	57
Capital expenditures		$ 93	$ 68	$ 226	$ 194

[1] (a)Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.

[2] (b)Ameren Transmission earnings include an allocation of financing costs from Ameren (parent).

[3] (a)Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.