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Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%	21.00%
Increases (decreases) from:
Amortization of excess deferred taxes	(7.00%)	(9.00%)	(7.00%)	(8.00%)
Valuation allowance	(7.00%)	0.00%	(3.00%)	0.00%
Amortization of deferred investment tax credit	(1.00%)	0.00%	(1.00%)	0.00%
Renewable and other tax credits	(3.00%)	(4.00%)	(3.00%)	(5.00%)
State tax	5.00%	4.00%	5.00%	5.00%
Depreciation differences	(1.00%)	0.00%	(1.00%)	0.00%
Other permanent items	0.00%	(1.00%)
Effective income tax rate	7.00%	11.00%	11.00%	13.00%
Income Tax Expense (Benefit)	$ 52	$ 57	$ 145	$ 140
TCJA - Private Letter Ruling
Increases (decreases) from:
Income Tax Expense (Benefit)			$ 48
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%	21.00%
Increases (decreases) from:
Amortization of excess deferred taxes	(9.00%)	(15.00%)	(9.00%)	(14.00%)
Valuation allowance	0.00%	0.00%	0.00%	0.00%
Amortization of deferred investment tax credit	(1.00%)	0.00%	(1.00%)	0.00%
Renewable and other tax credits	(7.00%)	(10.00%)	(7.00%)	(10.00%)
State tax	3.00%	3.00%	3.00%	3.00%
Depreciation differences	(1.00%)	(1.00%)	(1.00%)	(1.00%)
Other permanent items	0.00%	0.00%
Effective income tax rate	6.00%	(2.00%)	6.00%	(1.00%)
Income Tax Expense (Benefit)	$ 32	$ (6)	$ 43	$ (4)
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate	21.00%	21.00%	21.00%	21.00%
Increases (decreases) from:
Amortization of excess deferred taxes	(4.00%)	(4.00%)	(4.00%)	(4.00%)
Valuation allowance	(14.00%)	0.00%	(4.00%)	0.00%
Amortization of deferred investment tax credit	0.00%	0.00%	0.00%	0.00%
Renewable and other tax credits	0.00%	0.00%	0.00%	0.00%
State tax	7.00%	7.00%	7.00%	7.00%
Depreciation differences	(1.00%)	0.00%	0.00%	0.00%
Other permanent items	1.00%	0.00%
Effective income tax rate	10.00%	24.00%	20.00%	24.00%
Income Tax Expense (Benefit)	$ 17	$ 37	$ 134	$ 144
Ameren Illinois Company | TCJA - Private Letter Ruling
Increases (decreases) from:
Income Tax Expense (Benefit)			$ 23

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