Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment			4
External revenues	$ 1,563	$ 1,537	$ 3,148	$ 3,052
Intersegment revenues	0	0	0	0
Net Income (Loss) Available to Common Stockholders, Basic	239	193	390	295
Capital expenditures	533	494	$ 1,112	998
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	578	576	$ 1,338	1,279
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	62	57	157	136
Capital expenditures	302	257	602	484
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues	946	922	1,730	1,695
Intersegment revenues	9	12	17	30
Capital expenditures	205	159	454	355
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	386	387	785	771
Intersegment revenues	1	2	2	3
Net Income Available to Common Shareholder	33	33	66	63
Capital expenditures	132	122	254	242
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	142	134	453	398
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	7	5	49	38
Capital expenditures	66	58	126	109
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	89	92	180	188
Intersegment revenues	14	13	27	19
Net Income Available to Common Shareholder	36	34	73	68
Capital expenditures	130	156	275	290
Operating Segments | Other
Segment Reporting Information [Line Items]
External revenues	0	2	0	0
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	(5)	1	(4)	1
Capital expenditures	(2)	1	5	5
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	387	389	787	774
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	33	33	66	63
Capital expenditures	132	122	254	242
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	142	134	453	398
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	7	5	49	38
Capital expenditures	66	58	126	109
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	49	53	98	107
Intersegment revenues	13	12	26	18
Net Income Available to Common Shareholder	22	19	42	35
Capital expenditures	104	77	222	133
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(24)	(27)	(46)	(52)
Net Income Available to Common Shareholder	0	0	0	0
Capital expenditures	2	(2)	(2)	(3)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(13)	(12)	(26)	(18)
Net Income Available to Common Shareholder	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0