Income Taxes (Schedule of Cash Flow, Supplemental Disclosures) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Income Taxes [Line Items]
Federal	$ (309)	$ (92)	$ (37)
Total taxes paid	(312)	(92)	(24)
Proceeds from Income Tax Refunds	314	95	49
ILLINOIS
Income Taxes [Line Items]
State	34	(12)	28
MISSOURI
Income Taxes [Line Items]
State	(37)	12	(15)
Ameren Missouri
Income Taxes [Line Items]
Federal	(350)	(131)	(24)
Total taxes paid	(359)	(136)	(19)
Proceeds from Income Tax Refunds	314	95	49
Ameren Missouri \| ILLINOIS
Income Taxes [Line Items]
State	0	0	0
Ameren Missouri \| MISSOURI
Income Taxes [Line Items]
State	(9)	(5)	5
Ameren Illinois
Income Taxes [Line Items]
Federal	79	(25)	76
Total taxes paid	118	(46)	102
Ameren Illinois \| ILLINOIS
Income Taxes [Line Items]
State	39	(21)	26
Ameren Illinois \| MISSOURI
Income Taxes [Line Items]
State	$ 0	$ 0	$ 0

[Line Items] for Income Taxes [Table]

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Amount, after income tax refund received, of cash paid to state and local tax jurisdictions for tax on income.

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Amount, after refund, of cash paid to foreign, federal, state, and local jurisdictions as income tax.

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