Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	$ 167	$ 145	$ 332	$ 284
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount	45	33	86	73
Effective Income Tax Rate Reconciliation, Tax Credit, Energy-Related, Amount	34	22	67	50
Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount	(3)	(2)	(6)	(7)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount	0	1	(4)	(1)
Effective Income Tax Rate Reconciliation, Other Adjustments, Amortization Of Excess Deferred Income Taxes. Amount	(46)	(45)	(88)	(92)
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount	0	(48)	0	(48)
Effective Income Tax Rate Reconciliation, Other Adjustments, Depreciation Differences. Amount	(10)	(10)	(22)	(14)
Income Tax Expense (Benefit), Total	$ 119	$ 52	$ 231	$ 145
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%	21.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	5.00%		5.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Energy-Related, Percent	(4.00%)	(3.00%)	(4.00%)	(3.00%)
Effective Income Tax Rate Reconciliation, Tax Credit, Other, Percent	(1.00%)	(1.00%)	(1.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Percent	0.00%	0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Amortization Of Excess Deferred Income Taxes. Percent	(0.05)	(0.07)	(0.06)	(0.07)
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Percent	0.00%	(7.00%)	0.00%	(3.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Depreciation Differences. Percent	(0.01)	(0.01)	(0.01)	(0.01)
Effective Income Tax Rate Reconciliation, Percent, Total	15.00%	7.00%	14.00%	11.00%
Tax Jurisdiction of Domicile [Extensible Enumeration]			UNITED STATES
Private Letter Ruling
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income Tax Expense (Benefit), Total				$ 48
Ameren (parent)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		5.00%		5.00%
Union Electric Company
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	$ 125	$ 116	$ 175	$ 159
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount	17	16	23	22
Effective Income Tax Rate Reconciliation, Tax Credit, Energy-Related, Amount	55	42	76	55
Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount	(3)	(2)	(5)	(6)
Effective Income Tax Rate Reconciliation, Other Adjustments, Amortization Of Excess Deferred Income Taxes. Amount	(63)	(51)	(87)	(70)
Effective Income Tax Rate Reconciliation, Other Adjustments, Depreciation Differences. Amount	(10)	(5)	(15)	(7)
Income Tax Expense (Benefit), Total	$ 11	$ 32	$ 15	$ 43
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%	21.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	3.00%	3.00%	3.00%	3.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Energy-Related, Percent	(9.00%)	(7.00%)	(8.00%)	(7.00%)
Effective Income Tax Rate Reconciliation, Tax Credit, Other, Percent	(1.00%)	(1.00%)	(2.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Amortization Of Excess Deferred Income Taxes. Percent	(0.10)	(0.09)	(0.10)	(0.09)
Effective Income Tax Rate Reconciliation, Other Adjustments, Depreciation Differences. Percent	(0.02)	(0.01)	(0.02)	(0.01)
Effective Income Tax Rate Reconciliation, Percent, Total	2.00%	6.00%	2.00%	6.00%
Ameren Illinois Company
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	$ 42	$ 35	$ 157	$ 138
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount	15	12	57	48
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount	0	2	(1)	2
Effective Income Tax Rate Reconciliation, Other Adjustments, Amortization Of Excess Deferred Income Taxes. Amount	(4)	(6)	(11)	(24)
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount	0	(23)	0	(23)
Effective Income Tax Rate Reconciliation, Other Adjustments, Depreciation Differences. Amount	0	(3)	(5)	(7)
Income Tax Expense (Benefit), Total	$ 53	$ 17	$ 197	$ 134
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%	21.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	8.00%	7.00%	7.00%	7.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Percent	0.00%	1.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Amortization Of Excess Deferred Income Taxes. Percent	(0.02)	(0.04)	(0.01)	(0.04)
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Percent	0.00%	(14.00%)	0.00%	(4.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Depreciation Differences. Percent	0	(0.01)	(0.01)	0
Effective Income Tax Rate Reconciliation, Percent, Total	27.00%	10.00%	26.00%	20.00%
Ameren Illinois Company \| Private Letter Ruling
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income Tax Expense (Benefit), Total				$ 23

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