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Summary Of Significant Accounting Policies (Narrative) (Details) customer in Millions	12 Months Ended
	Dec. 31, 2025 USD ($) mi² customer segment	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Accounting Policies [Line Items]
Goodwill	$ 411,000,000	$ 411,000,000
Number of reportable segments | segment	4
Goodwill, Impairment Loss	$ 0
Noncontrolling Interest in Variable Interest Entity	64,000,000	74,000,000
Cash Surrender Value of Life Insurance	219,000,000	260,000,000
Loans, Gross, Insurance Policy	$ 117,000,000	110,000,000
Ameren Missouri
Accounting Policies [Line Items]
Public Utilities, Area Serviced | mi²	24,000
Ameren Illinois
Accounting Policies [Line Items]
Public Utilities, Area Serviced | mi²	43,700
Goodwill	$ 411,000,000	411,000,000
Number of reportable segments | segment	3
Goodwill, Impairment Loss	$ 0
Cash Surrender Value of Life Insurance	126,000,000	118,000,000
Loans, Gross, Insurance Policy	$ 117,000,000	$ 110,000,000
Minimum
Accounting Policies [Line Items]
Percent of average depreciable cost	3.00%	3.00%	3.00%
Maximum
Accounting Policies [Line Items]
Percent of average depreciable cost	4.00%	4.00%	4.00%
Electric | Ameren Missouri
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer	1.3
Power | Ameren Illinois
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer	1.2
Natural gas | Ameren Missouri
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer	0.1
Natural gas | Ameren Illinois
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer	0.8
Ameren Illinois Electric Distribution
Accounting Policies [Line Items]
Goodwill	$ 238,000,000
Ameren Illinois Gas
Accounting Policies [Line Items]
Goodwill	80,000,000
Ameren Illinois Transmission
Accounting Policies [Line Items]
Goodwill	93,000,000
Ameren Transmission
Accounting Policies [Line Items]
Goodwill	93,000,000
AROs | Ameren Missouri
Accounting Policies [Line Items]
Noncash Depreciation related to ARO	7,000,000	$ 2,000,000	$ 9,000,000
Partnership Funding Commitment [Member]
Accounting Policies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 28,000,000

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