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Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Derivative [Line Items] |  |
Derivative Assets | $ 11 | $ 22
Derivative Liability | 145 | 168
Fuel oils | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 2 | 2
Fuel oils | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 1 | 1
Fuel oils | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 0 | 1
Natural gas | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 2 | 0
Natural gas | Other assets |  |
Derivative [Line Items] |  |
Derivative Assets | 5 | 6
Natural gas | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 54 | 57
Natural gas | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 36 | 40
Power | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 2 | 5
Power | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 11 | 13
Power | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 43 | 56
Uranium | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 0 | 9
Union Electric Company |  |
Derivative [Line Items] |  |
Derivative Assets | 7 | 19
Derivative Liability | 20 | 25
Union Electric Company | Fuel oils | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 2 | 2
Union Electric Company | Fuel oils | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 1 | 1
Union Electric Company | Fuel oils | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 0 | 1
Union Electric Company | Natural gas | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 0 | 0
Union Electric Company | Natural gas | Other assets |  |
Derivative [Line Items] |  |
Derivative Assets | 3 | 3
Union Electric Company | Natural gas | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 11 | 12
Union Electric Company | Natural gas | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 8 | 10
Union Electric Company | Power | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 2 | 5
Union Electric Company | Power | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 0 | 1
Union Electric Company | Power | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 0 | 0
Union Electric Company | Uranium | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 0 | 9
Ameren Illinois Company |  |
Derivative [Line Items] |  |
Derivative Assets | 4 | 3
Derivative Liability | 125 | 143
Ameren Illinois Company | Fuel oils | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 0 | 0
Ameren Illinois Company | Fuel oils | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 0 | 0
Ameren Illinois Company | Fuel oils | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 0 | 0
Ameren Illinois Company | Natural gas | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 2 | 0
Ameren Illinois Company | Natural gas | Other assets |  |
Derivative [Line Items] |  |
Derivative Assets | 2 | 3
Ameren Illinois Company | Natural gas | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 43 | 45
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 28 | 30
Ameren Illinois Company | Power | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | 0 | 0
Ameren Illinois Company | Power | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 11 | 12
Ameren Illinois Company | Power | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 43 | 56
Ameren Illinois Company | Uranium | Other current assets |  |
Derivative [Line Items] |  |
Derivative Assets | $ 0 | $ 0

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