Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment			4
External revenues	$ 1,724	$ 1,723	$ 4,872	$ 4,775
Intersegment revenues	0	0	0	0
Net Income (Loss) Available to Common Stockholders, Basic	357	288	747	583
Capital expenditures	577	525	$ 1,689	1,523
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	564	574	$ 1,902	1,853
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	63	55	220	191
Capital expenditures	345	276	947	760
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues	1,118	1,105	2,848	2,800
Intersegment revenues	11	11	28	41
Net Income Available to Common Shareholder	294	234	500	359
Capital expenditures	210	178	664	533
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	392	404	1,177	1,175
Intersegment revenues	0	0	2	3
Net Income Available to Common Shareholder	35	31	101	94
Capital expenditures	135	112	389	354
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	116	111	569	509
Intersegment revenues	0	1	0	1
Net Income Available to Common Shareholder	0	2	49	40
Capital expenditures	111	71	237	180
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	98	105	278	293
Intersegment revenues	15	14	42	33
Net Income Available to Common Shareholder	48	38	121	106
Capital expenditures	124	173	399	463
Operating Segments | Other
Segment Reporting Information [Line Items]
External revenues	0	(2)	0	(2)
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	(20)	(17)	(24)	(16)
Capital expenditures	1	(2)	6	3
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	392	404	1,179	1,178
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	35	31	101	94
Capital expenditures	135	112	389	354
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	116	112	569	510
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	0	2	49	40
Capital expenditures	111	71	237	180
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	56	58	154	165
Intersegment revenues	15	14	41	32
Net Income Available to Common Shareholder	28	22	70	57
Capital expenditures	99	93	321	226
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(26)	(26)	(72)	(78)
Net Income Available to Common Shareholder	0	0	0	0
Capital expenditures	(4)	(7)	(6)	(10)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(15)	(14)	(41)	(32)
Net Income Available to Common Shareholder	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0