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v3.25.4

Summary Of Significant Accounting Policies (Narrative) (Details) customer in Millions | 12 Months Ended
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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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