Segment Information (Schedule Of Segment Reporting Information By Segment) (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment									4
Revenues	$ 1,419	$ 1,724	$ 1,563	$ 1,585	$ 1,399	$ 1,723	$ 1,537	$ 1,515	$ 6,291	$ 6,174	$ 6,076
Intersegment revenues									0	0	0
Depreciation and amortization									955	896	845
Interest income									33	34	40
Interest charges									401	391	382
Income taxes (benefit)									237	576	382
Net Income (Loss) Available to Common Stockholders, Basic	68	357	239	151	(60)	288	193	102	815	523	653
Capital expenditures									$ 2,286	2,132	2,076
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]
Revenues	674	564	578	760	674	574	576	703	$ 2,576	2,527	2,489
Intersegment revenues									0	0	0
Depreciation and amortization									374	341	319
Interest income									6	7	12
Interest charges									149	144	140
Income taxes (benefit)									98	166	158
Net Income (Loss) Attributable to Parent									304	268	252
Net Income (Loss) Available to Common Stockholders, Basic	$ 84	$ 63	$ 62	$ 95	$ 77	$ 55	$ 57	$ 79
Capital expenditures									1,258	1,076	924
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
Revenues									3,555	3,488	3,470
Intersegment revenues									34	49	54
Depreciation and amortization									550	533	514
Interest income									28	27	28
Interest charges									200	207	211
Income taxes (benefit)									124	254	216
Net Income (Loss) Attributable to Parent									478	323	357
Capital expenditures									914	773	738
Operating Segments | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
Revenues									1,544	1,564	1,544
Intersegment revenues									3	4	4
Depreciation and amortization									259	239	226
Interest income									6	7	11
Interest charges									73	73	72
Income taxes (benefit)									41	83	78
Net Income (Loss) Attributable to Parent									136	131	126
Capital expenditures									503	476	470
Operating Segments | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
Revenues									814	742	753
Intersegment revenues									1	1	1
Depreciation and amortization									65	59	55
Interest income									0	0	0
Interest charges									38	36	34
Income taxes (benefit)									25	36	39
Net Income (Loss) Attributable to Parent									70	60	59
Capital expenditures									311	245	181
Operating Segments | Ameren Transmission [Member]
Segment Reporting Information [Line Items]
Revenues									378	382	309
Intersegment revenues									55	44	46
Depreciation and amortization									77	60	43
Interest income									0	0	1
Interest charges									75	67	58
Income taxes (benefit)									56	90	74
Net Income (Loss) Attributable to Parent									164	140	117
Capital expenditures									562	644	689
Operating Segments | Other Segment
Segment Reporting Information [Line Items]
Revenues									0	(2)	0
Intersegment revenues									0	0	0
Depreciation and amortization									4	5	7
Interest income									4	11	11
Interest charges									19	19	18
Income taxes (benefit)									(9)	113	(25)
Net Income (Loss) Attributable to Parent									(33)	(131)	(6)
Capital expenditures									5	1	4
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
Revenues									1,547	1,568	1,548
Intersegment revenues									0	0	0
Depreciation and amortization									259	239	226
Interest income									6	7	11
Interest charges									73	73	72
Income taxes (benefit)									41	83	78
Net Income (Loss) Attributable to Parent									136	131	126
Capital expenditures									503	476	470
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
Revenues									815	743	754
Intersegment revenues									0	0	0
Depreciation and amortization									65	59	55
Interest income									0	0	0
Interest charges									38	36	34
Income taxes (benefit)									25	36	39
Net Income (Loss) Attributable to Parent									70	60	59
Capital expenditures									311	245	181
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Segment Reporting Information [Line Items]
Revenues									214	216	187
Intersegment revenues									53	42	45
Depreciation and amortization									50	43	38
Interest income									0	0	1
Interest charges									38	35	34
Income taxes (benefit)									32	47	41
Net Income (Loss) Attributable to Parent									98	77	67
Capital expenditures									444	355	273
Intersegment Elimination
Segment Reporting Information [Line Items]
Revenues									0	0	0
Intersegment revenues									(93)	(98)	(105)
Depreciation and amortization									0	0	0
Interest income									(5)	(11)	(11)
Interest charges									(4)	(11)	(11)
Income taxes (benefit)									0	0	0
Net Income (Loss) Attributable to Parent									0	0	0
Capital expenditures									(9)	(7)	(6)
Intersegment Elimination | Ameren Illinois Company
Segment Reporting Information [Line Items]
Revenues									0	0	0
Intersegment revenues									(53)	(42)	(45)
Depreciation and amortization									0	0	0
Interest income									0	0	0
Interest charges									0	0	0
Income taxes (benefit)									0	0	0
Net Income (Loss) Attributable to Parent									0	0	0
Capital expenditures									$ 0	$ 0	$ 0