Consolidated Statement of Income (Loss) and Comprehensive Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Operating Revenues:
Total operating revenues	$ 1,472	$ 1,398	$ 3,038	$ 2,838
Operating Expenses:
Fuel	173	119	238	259
Purchased power	129	109	320	243
Natural gas purchased for resale	65	42	230	149
Other operations and maintenance	412	384	832	822
Depreciation and amortization	285	271	566	526
Taxes other than income taxes	122	119	250	244
Total operating expenses	1,186	1,044	2,436	2,243
Operating Income	286	354	602	595
Other Income, Net	49	48	95	69
Interest Charges	96	108	196	201
Income Before Income Taxes	239	294	501	463
Income Taxes	31	50	58	71
Net Income	208	244	443	392
Less: Net Income Attributable to Noncontrolling Interests	1	1	3	3
Net Income Attributable to Ameren Common Shareholders	207	243	440	389
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	(1)	0	0	1
Comprehensive Income	207	244	443	393
Less: Comprehensive Income Attributable to Noncontrolling Interests	1	1	3	3
Comprehensive Income Attributable to Ameren Common Shareholders	$ 206	$ 243	$ 440	$ 390
Earnings Per Share, Basic and Diluted [Abstract]
Earnings per Common Share - Basic (in dollars per share)	$ 0.81	$ 0.99	$ 1.72	$ 1.58
Earnings per Common Share - Diluted (in dollars per share)	$ 0.80	$ 0.98	$ 1.71	$ 1.57
Weighted Average Common Shares Outstanding - Basic (in shares)	256.1	246.9	255.2	246.7
Weighted Average Number of Shares Outstanding - Diluted (in shares)	257.2	247.9	256.5	248.0
Electricity
Operating Revenues:
Total operating revenues	$ 1,284	$ 1,237	$ 2,440	$ 2,357
Natural Gas
Operating Revenues:
Total operating revenues	188	161	598	481
Union Electric Company
Operating Revenues:
Total operating revenues	809	792	1,513	1,472
Operating Expenses:
Fuel	173	119	238	259
Purchased power	50	37	138	76
Natural gas purchased for resale	5	6	36	24
Other operations and maintenance	218	202	443	441
Depreciation and amortization	157	155	313	294
Taxes other than income taxes	85	83	162	162
Total operating expenses	688	602	1,330	1,256
Operating Income	121	190	183	216
Other Income, Net	24	25	47	29
Interest Charges	36	50	75	90
Income Before Income Taxes	109	165	155	155
Income Taxes	(3)	12	(5)	11
Net Income	112	153	160	144
Preferred Stock Dividends	1	1	2	2
Net Income (Loss) Attributable to Parent	111	152	158	142
Union Electric Company | Electricity
Operating Revenues:
Total operating revenues	789	771	1,430	1,402
Union Electric Company | Natural Gas
Operating Revenues:
Total operating revenues	20	21	83	70
Ameren Illinois Company
Operating Revenues:
Total operating revenues	629	567	1,452	1,290
Operating Expenses:
Purchased power	84	76	190	174
Natural gas purchased for resale	60	36	194	125
Other operations and maintenance	193	182	387	381
Depreciation and amortization	117	107	232	214
Taxes other than income taxes	34	32	80	74
Total operating expenses	488	433	1,083	968
Operating Income	141	134	369	322
Other Income, Net	16	17	30	28
Interest Charges	40	38	82	77
Income Before Income Taxes	117	113	317	273
Income Taxes	31	29	81	68
Net Income	86	84	236	205
Preferred Stock Dividends	0	1	1	2
Net Income (Loss) Attributable to Parent	86	83	235	203
Ameren Illinois Company | Electricity
Operating Revenues:
Total operating revenues	461	427	937	879
Ameren Illinois Company | Natural Gas
Operating Revenues:
Total operating revenues	$ 168	$ 140	$ 515	$ 411