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Segment Information (Schedule Of Segment Reporting Information By Segment) (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| segment									4
External revenues	$ 1,316	$ 1,659	$ 1,379	$ 1,556	$ 1,419	$ 1,724	$ 1,563	$ 1,585	$ 5,910	$ 6,291	$ 6,174
Depreciation and amortization									995	955	896
Interest income									33	33	34
Interest charges									381	401	391
Income taxes (benefit)									182	237	576
Net Income (Loss) Available to Common Stockholders, Basic	94	364	179	191	68	357	239	151	828	815	523
Capital expenditures									2,411	2,286	2,132
Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues									5,891	6,291	6,174
Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues									$ 19	0	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	654	564	547	762	674	564	578	760	$ 2,527	2,576	2,527
Depreciation and amortization									406	374	341
Interest income									6	6	7
Interest charges									147	149	144
Income taxes (benefit)									110	98	166
Net Income Available to Common Shareholder									343	304	268
Net Income (Loss) Available to Common Stockholders, Basic	$ 96	$ 65	$ 62	$ 120	$ 84	$ 63	$ 62	$ 95
Capital expenditures									1,208	1,258	1,076
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									2,527	2,576	2,527
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									3,243	3,589	3,537
Depreciation and amortization									556	550	533
Interest income									26	28	27
Interest charges									178	200	207
Income taxes (benefit)									68	124	254
Net Income Available to Common Shareholder									426	478	323
Capital expenditures									1,076	914	773
Operating Segments \| Union Electric Company \| Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues									3,212	3,555	3,488
Operating Segments \| Union Electric Company \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues									31	34	49
Operating Segments \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues									1,504	1,547	1,568
Depreciation and amortization									273	259	239
Interest income									6	6	7
Interest charges									71	73	73
Income taxes (benefit)									45	41	83
Net Income Available to Common Shareholder									146	136	131
Capital expenditures									518	503	476
Operating Segments \| Ameren Illinois Electric Distribution \| Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues									1,487	1,544	1,564
Operating Segments \| Ameren Illinois Electric Distribution \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues									17	3	4
Operating Segments \| Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues									797	815	743
Depreciation and amortization									78	65	59
Interest income									0	0	0
Interest charges									38	38	36
Income taxes (benefit)									30	25	36
Net Income Available to Common Shareholder									84	70	60
Capital expenditures									318	311	245
Operating Segments \| Ameren Illinois Gas \| Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues									791	814	742
Operating Segments \| Ameren Illinois Gas \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues									6	1	1
Operating Segments \| Ameren Transmission
Segment Reporting Information [Line Items]
External revenues									464	433	426
Depreciation and amortization									84	77	60
Interest income									1	0	0
Interest charges									74	75	67
Income taxes (benefit)									64	56	90
Net Income Available to Common Shareholder									185	164	140
Capital expenditures									528	562	644
Operating Segments \| Ameren Transmission \| Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues									401	378	382
Operating Segments \| Ameren Transmission \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues									63	55	44
Operating Segments \| Other Segment
Segment Reporting Information [Line Items]
External revenues									0	0	(2)
Depreciation and amortization									4	4	5
Interest income									5	4	11
Interest charges									25	19	19
Income taxes (benefit)									(25)	(9)	113
Net Income Available to Common Shareholder									(13)	(33)	(131)
Capital expenditures									3	5	1
Operating Segments \| Other Segment \| Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues									0	0	(2)
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									273	259	239
Interest income									6	6	7
Interest charges									71	73	73
Income taxes (benefit)									45	41	83
Net Income Available to Common Shareholder									146	136	131
Capital expenditures									518	503	476
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution \| Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues									1,504	1,547	1,568
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									78	65	59
Interest income									0	0	0
Interest charges									38	38	36
Income taxes (benefit)									30	25	36
Net Income Available to Common Shareholder									84	70	60
Capital expenditures									318	311	245
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Gas \| Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues									797	815	743
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									55	50	43
Interest income									0	0	0
Interest charges									38	38	35
Income taxes (benefit)									35	32	47
Net Income Available to Common Shareholder									113	98	77
Capital expenditures									372	444	355
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission \| Reportable Subsegments
Segment Reporting Information [Line Items]
External revenues									226	214	216
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues									62	53	42
Intersegment Elimination
Segment Reporting Information [Line Items]
External revenues									(98)	(93)	(98)
Depreciation and amortization									0	0	0
Interest income									(5)	(5)	(11)
Interest charges									(5)	(4)	(11)
Income taxes (benefit)									0	0	0
Net Income Available to Common Shareholder									0	0	0
Capital expenditures									(32)	(9)	(7)
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									(98)	(93)	(98)
Intersegment Elimination \| Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues									(62)	(53)	(42)
Depreciation and amortization									0	0	0
Interest income									0	0	0
Interest charges									0	0	0
Income taxes (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									$ (62)	$ (53)	$ (42)