Segment Information (Schedule Of Segment Reporting Information By Segment) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	4
External revenues	$ 5,794	$ 5,910	$ 6,291
Depreciation and amortization	1,075	995	955
Interest income	29	33	33
Interest Charges	419	381	401
Income tax benefit	155	182	237
Net Income (Loss) Available to Common Stockholders, Basic	871	828	815
Capital expenditures	3,233	2,411	2,286
Wind generation expenditures	564	0	0
Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	5,794	5,891	6,291
Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	$ 0	19	0
Union Electric Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment	1
Ameren Illinois Electric Distribution | Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues		14
Ameren Illinois Gas | Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues		5
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment	3
External revenues	$ 2,535	2,527	2,576
Depreciation and amortization	434	406	374
Interest income	3	6	6
Interest Charges	155	147	149
Income tax benefit	124	110	98
Net Income Available to Common Shareholder	379	343	304
Capital expenditures	1,447	1,208	1,258
Ameren Illinois Company | Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues		19
Ameren Illinois Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	2,535	2,527	2,576
Ameren Illinois Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues	3,109	3,243	3,589
Depreciation and amortization	604	556	550
Interest income	26	26	28
Interest Charges	190	178	200
Income tax benefit	34	68	124
Net Income Available to Common Shareholder	436	426	478
Capital expenditures	1,666	1,076	914
Operating Segments | Union Electric Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	3,069	3,212	3,555
Operating Segments | Union Electric Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	40	31	34
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	1,498	1,504	1,547
Depreciation and amortization	288	273	259
Interest income	2	6	6
Interest Charges	72	71	73
Income tax benefit	42	45	41
Net Income Available to Common Shareholder	143	146	136
Capital expenditures	543	518	503
Operating Segments | Ameren Illinois Electric Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	1,496	1,487	1,544
Operating Segments | Ameren Illinois Electric Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	2	17	3
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	760	797	815
Depreciation and amortization	81	78	65
Interest income	0	0	0
Interest Charges	41	38	38
Income tax benefit	36	30	25
Net Income Available to Common Shareholder	99	84	70
Capital expenditures	301	318	311
Operating Segments | Ameren Illinois Gas | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	760	791	814
Operating Segments | Ameren Illinois Gas | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	6	1
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	523	464	433
Depreciation and amortization	98	84	77
Interest income	1	1	0
Interest Charges	78	74	75
Income tax benefit	78	64	56
Net Income Available to Common Shareholder	216	185	164
Capital expenditures	716	528	562
Operating Segments | Ameren Transmission | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	469	401	378
Operating Segments | Ameren Transmission | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	54	63	55
Operating Segments | Other Segment
Segment Reporting Information [Line Items]
Depreciation and amortization	4	4	4
Interest income	4	5	4
Interest Charges	42	25	19
Income tax benefit	(35)	(25)	(9)
Net Income Available to Common Shareholder	(23)	(13)	(33)
Capital expenditures	5	3	5
Operating Segments | Other Segment | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	0	0	0
Operating Segments | Other Segment | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Depreciation and amortization	288	273	259
Interest income	2	6	6
Interest Charges	72	71	73
Income tax benefit	42	45	41
Net Income Available to Common Shareholder	143	146	136
Capital expenditures	543	518	503
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	1,498	1,504	1,547
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
Depreciation and amortization	81	78	65
Interest income	0	0	0
Interest Charges	41	38	38
Income tax benefit	36	30	25
Net Income Available to Common Shareholder	99	84	70
Capital expenditures	301	318	311
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	760	797	815
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Depreciation and amortization	65	55	50
Interest income	1	0	0
Interest Charges	42	38	38
Income tax benefit	46	35	32
Net Income Available to Common Shareholder	137	113	98
Capital expenditures	603	372	444
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	277	226	214
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	52	62	53
Intersegment Elimination
Segment Reporting Information [Line Items]
External revenues	(96)	(98)	(93)
Depreciation and amortization	0	0	0
Interest income	(4)	(5)	(5)
Interest Charges	(4)	(5)	(4)
Income tax benefit	0	0	0
Net Income Available to Common Shareholder	0	0	0
Capital expenditures	2	(32)	(9)
Intersegment Elimination | Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Capital expenditures		24
Intersegment Elimination | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	0	0	0
Intersegment Elimination | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	(96)	(98)	(93)
Intersegment Elimination | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	(52)	(62)	(53)
Depreciation and amortization	0	0	0
Interest income	0	0	0
Interest Charges	0	0	0
Income tax benefit	0	0	0
Net Income Available to Common Shareholder	0	0	0
Capital expenditures	0	0	0
Intersegment Elimination | Ameren Illinois Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues	0	0	0
Intersegment Elimination | Ameren Illinois Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	$ (52)	$ (62)	$ (53)