Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment				4
External revenues		$ 1,538	$ 1,427	$ 3,052	$ 2,861
Intersegment revenues		0	0	0	0
Net Income Attributable to Ameren Common Shareholders		193	147	295	252
Capital expenditures		494	504	$ 998	1,000
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		576	542	$ 1,279	1,219
Intersegment revenues		0	0	0
Net Income Available to Common Shareholder		57	45	136	104
Capital expenditures		257	231	484	442
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues		0	0	0	0
Intersegment revenues		(27)	(22)	(52)	(49)
Net Income Available to Common Shareholder		0	0	0	0
Capital expenditures		(2)	0	(3)	0
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues		0	0	0
Intersegment revenues		(12)	(10)	(18)	(21)
Net Income Available to Common Shareholder		0	0	0
Capital expenditures		0	0	0
Operating Segments [Member] | Union Electric Company
Segment Reporting Information [Line Items]
External revenues		923	857	1,695	1,583
Intersegment revenues		12	10	30	25
Net Income Available to Common Shareholder		120	92	125	106
Capital expenditures		159	175	355	353
Operating Segments [Member] | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues		387	357	771	708
Intersegment revenues		2	1	3	2
Net Income Available to Common Shareholder		33	18	63	29
Capital expenditures		122	119	242	236
Operating Segments [Member] | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues		134	131	398	416
Intersegment revenues		0	0	0	0
Net Income Available to Common Shareholder		5	7	38	42
Capital expenditures		58	45	109	80
Operating Segments [Member] | Ameren Transmission [Member]
Segment Reporting Information [Line Items]
External revenues		92	81	188	153
Intersegment revenues	[1]	13	11	19	22
Net Income Available to Common Shareholder	[2]	34	32	68	59
Capital expenditures		156	164	290	328
Operating Segments [Member] | Other
Segment Reporting Information [Line Items]
External revenues		2	1	0	1
Intersegment revenues		0	0	0	0
Net Income Available to Common Shareholder		1	(2)	1	16
Capital expenditures		1	1	5	3
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues		389	358	774	710
Intersegment revenues		0	0	0
Net Income Available to Common Shareholder		33	18	63	29
Capital expenditures		122	119	242	236
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues		134	131	398	416
Intersegment revenues		0	0	0
Net Income Available to Common Shareholder		5	7	38	42
Capital expenditures		58	45	109	80
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Segment Reporting Information [Line Items]
External revenues		53	53	107	93
Intersegment revenues	[3]	12	10	18	21
Net Income Available to Common Shareholder		19	20	35	33
Capital expenditures		$ 77	$ 67	$ 133	$ 126

[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.