Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment			4
External revenues	$ 1,379	$ 1,563	$ 2,935	$ 3,148
Intersegment revenues	0	0	0	0
Net Income (Loss) Available to Common Stockholders, Basic	179	239	370	390
Capital expenditures	581	533	$ 1,125	1,112
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	547	578	$ 1,309	1,338
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	62	62	182	157
Capital expenditures	289	302	556	602
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues	790	946	1,541	1,730
Intersegment revenues	8	9	15	17
Net Income Available to Common Shareholder	107	168	146	206
Capital expenditures	255	205	495	454
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	358	386	744	785
Intersegment revenues	1	1	2	2
Net Income Available to Common Shareholder	37	33	73	66
Capital expenditures	127	132	251	254
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	136	142	456	453
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	1	7	58	49
Capital expenditures	77	66	128	126
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	95	89	194	180
Intersegment revenues	14	14	29	27
Net Income Available to Common Shareholder	42	36	86	73
Capital expenditures	127	130	248	275
Operating Segments | Other
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	(8)	(5)	7	(4)
Capital expenditures	0	(2)	10	5
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	359	387	746	787
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	37	33	73	66
Capital expenditures	127	132	251	254
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	136	142	456	453
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	1	7	58	49
Capital expenditures	77	66	128	126
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	52	49	107	98
Intersegment revenues	14	13	29	26
Net Income Available to Common Shareholder	24	22	51	42
Capital expenditures	85	104	177	222
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(23)	(24)	(46)	(46)
Net Income Available to Common Shareholder	0	0	0	0
Capital expenditures	(5)	2	(7)	(2)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(14)	(13)	(29)	(26)
Net Income Available to Common Shareholder	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0