Consolidated Statement of Income (Loss) and Comprehensive Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,628	$ 1,659	$ 4,466	$ 4,594
Operating Expenses:
Fuel	141	147	400	409
Purchased Power	140	148	383	440
Natural gas purchased for resale	34	31	183	236
Other operations and maintenance	418	434	1,240	1,301
Depreciation and amortization	273	248	799	745
Taxes other than income taxes	128	131	372	375
Total operating expenses	1,134	1,139	3,377	3,506
Operating Income	494	520	1,089	1,088
Other Income (Expense), Net	48	34	117	99
Interest Charges	110	96	311	290
Income Before Income Taxes	432	458	895	897
Income Taxes	63	92	134	158
Net income	369	366	761	739
Less: Net Income Attributable to Noncontrolling Interests	2	2	5	5
Net income attributable to Ameren common shareholders	367	364	756	734
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1	0	2	1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	370	366	763	740
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2	5	5
Comprehensive Income	$ 368	$ 364	$ 758	$ 735
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 1.48	$ 1.48	$ 3.06	$ 2.99
Earnings Per Share, Diluted	$ 1.47	$ 1.47	$ 3.04	$ 2.97
Average Common Shares Outstanding - Basic (in shares)	247.1	245.9	246.8	245.5
Weighted Average Number of Shares Outstanding, Diluted (in shares)	249.2	247.5	248.4	247.0
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,489	$ 1,528	$ 3,846	$ 3,928
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	139	131	620	666
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,001	1,059	2,473	2,615
Operating Expenses:
Fuel	141	147	400	409
Purchased Power	48	49	124	160
Natural gas purchased for resale	5	6	29	41
Other operations and maintenance	221	242	662	720
Depreciation and amortization	154	138	448	417
Taxes other than income taxes	92	96	254	256
Total operating expenses	661	678	1,917	2,003
Operating Income	340	381	556	612
Other Income (Expense), Net	26	15	55	43
Interest Charges	50	44	140	136
Income Before Income Taxes	316	352	471	519
Income Taxes	18	51	29	70
Net income	298	301	442	449
Preferred Stock Dividends	1	1	3	3
Net Income (Loss) Attributable to Parent	297	300	439	446
Union Electric Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	984	1,040	2,386	2,517
Union Electric Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	17	19	87	98
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	589	564	1,879	1,873
Operating Expenses:
Purchased Power	97	101	271	284
Natural gas purchased for resale	29	25	154	195
Other operations and maintenance	197	193	578	580
Depreciation and amortization	109	102	323	304
Taxes other than income taxes	33	33	107	110
Total operating expenses	465	454	1,433	1,473
Operating Income	124	110	446	400
Other Income (Expense), Net	17	13	45	39
Interest Charges	39	38	116	111
Income Before Income Taxes	102	85	375	328
Income Taxes	25	20	93	79
Net income	77	65	282	249
Preferred Stock Dividends	0	0	2	2
Net Income (Loss) Attributable to Parent	77	65	280	247
Ameren Illinois Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	467	452	1,346	1,305
Ameren Illinois Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 122	$ 112	$ 533	$ 568