Consolidated Statement of Income (Loss) and Comprehensive Income - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating Revenues:
Total operating revenues	$ 1,566	$ 1,440
Operating Expenses:
Fuel	65	140
Purchased power	191	134
Natural gas purchased for resale	165	107
Other operations and maintenance	420	438
Depreciation and amortization	281	255
Taxes other than income taxes	128	125
Total operating expenses	1,250	1,199
Operating Income	316	241
Other Income, Net	46	21
Interest Charges	100	93
Income Before Income Taxes	262	169
Income Taxes	27	21
Net Income	235	148
Less: Net Income Attributable to Noncontrolling Interests	2	2
Net Income Attributable to Ameren Common Shareholders	233	146
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1	1
Comprehensive Income	236	149
Less: Comprehensive Income Attributable to Noncontrolling Interests	2	2
Comprehensive Income Attributable to Ameren Common Shareholders	$ 234	$ 147
Earnings Per Share, Basic and Diluted [Abstract]
Earnings per Common Share - Basic (in dollars per share)	$ 0.92	$ 0.59
Earnings per Common Share - Diluted (in dollars per share)	$ 0.91	$ 0.59
Weighted Average Common Shares Outstanding - Basic (in shares)	254.4	246.4
Weighted Average Number of Shares Outstanding - Diluted (in shares)	255.9	248.1
Electricity
Operating Revenues:
Total operating revenues	$ 1,156	$ 1,120
Natural Gas
Operating Revenues:
Total operating revenues	410	320
Union Electric Company
Operating Revenues:
Total operating revenues	704	680
Operating Expenses:
Fuel	65	140
Purchased power	88	39
Natural gas purchased for resale	31	18
Other operations and maintenance	225	239
Depreciation and amortization	156	139
Taxes other than income taxes	77	79
Total operating expenses	642	654
Operating Income	62	26
Other Income, Net	23	4
Interest Charges	39	40
Income Before Income Taxes	46	(10)
Income Taxes	(2)	(1)
Net Income	48	(9)
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	47	(10)
Union Electric Company | Electricity
Operating Revenues:
Total operating revenues	641	631
Union Electric Company | Natural Gas
Operating Revenues:
Total operating revenues	63	49
Ameren Illinois Company
Operating Revenues:
Total operating revenues	823	723
Operating Expenses:
Purchased power	106	98
Natural gas purchased for resale	134	89
Other operations and maintenance	194	199
Depreciation and amortization	115	107
Taxes other than income taxes	46	42
Total operating expenses	595	535
Operating Income	228	188
Other Income, Net	14	11
Interest Charges	42	39
Income Before Income Taxes	200	160
Income Taxes	50	39
Net Income	150	121
Preferred Stock Dividends	1	1
Net Income (Loss) Attributable to Parent	149	120
Ameren Illinois Company | Electricity
Operating Revenues:
Total operating revenues	476	452
Ameren Illinois Company | Natural Gas
Operating Revenues:
Total operating revenues	$ 347	$ 271