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Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions | Dec. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Derivative [Line Items] |  |
Derivative assets | $ 16 | $ 16
Derivative liabilities | 126 | 103
Natural gas | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 2 | 4
Natural gas | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 3 | 6
Natural gas | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 26 | 27
Natural gas | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 16 | 19
Power | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 11 | 6
Power | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 24 | 10
Power | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 55 | 43
Fuel oils | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 3 | 2
Fuel oils | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 2 | 2
Ameren Missouri |  |
Derivative [Line Items] |  |
Derivative assets | 12 | 10
Derivative liabilities | 15 | 15
Ameren Missouri | Natural gas | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 1 | 2
Ameren Missouri | Natural gas | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 0 | 2
Ameren Missouri | Natural gas | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 5 | 5
Ameren Missouri | Natural gas | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 5 | 6
Ameren Missouri | Power | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 11 | 6
Ameren Missouri | Power | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 0 | 0
Ameren Missouri | Power | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 0 | 0
Ameren Missouri | Fuel oils | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 3 | 2
Ameren Missouri | Fuel oils | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 2 | 2
Ameren Illinois |  |
Derivative [Line Items] |  |
Derivative assets | 4 | 6
Derivative liabilities | 111 | 88
Ameren Illinois | Natural gas | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 1 | 2
Ameren Illinois | Natural gas | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 3 | 4
Ameren Illinois | Natural gas | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 21 | 22
Ameren Illinois | Natural gas | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 11 | 13
Ameren Illinois | Power | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 0 | 0
Ameren Illinois | Power | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 24 | 10
Ameren Illinois | Power | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 55 | 43
Ameren Illinois | Fuel oils | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | 0 | 0
Ameren Illinois | Fuel oils | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative liabilities | $ 0 | $ 0

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