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Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions | Jun. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Derivative [Line Items] |  |
Derivative assets | $ 43 | $ 89
Derivative Liability | 145 | 136
Fuel oils | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 5 | 13
Fuel oils | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 1 | 3
Fuel oils | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 1 | 0
Fuel oils | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 1 | 0
Natural gas | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 10 | 30
Natural gas | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 10 | 20
Natural gas | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 35 | 27
Natural gas | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 27 | 11
Power | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 15 | 16
Power | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 0 | 4
Power | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 23 | 61
Power | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 58 | 37
Uranium | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 2 | 2
Uranium | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 0 | 1
Union Electric Company |  |
Derivative [Line Items] |  |
Derivative assets | 30 | 49
Derivative Liability | 31 | 68
Union Electric Company | Fuel oils | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 5 | 13
Union Electric Company | Fuel oils | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 1 | 3
Union Electric Company | Fuel oils | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 1 | 0
Union Electric Company | Fuel oils | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 1 | 0
Union Electric Company | Natural gas | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 2 | 7
Union Electric Company | Natural gas | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 5 | 9
Union Electric Company | Natural gas | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 8 | 7
Union Electric Company | Natural gas | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 8 | 2
Union Electric Company | Power | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 15 | 14
Union Electric Company | Power | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 0 | 0
Union Electric Company | Power | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 13 | 59
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 | 2 | 2
Union Electric Company | Uranium | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 0 | 1
Ameren Illinois Company |  |
Derivative [Line Items] |  |
Derivative assets | 13 | 40
Derivative Liability | 114 | 68
Ameren Illinois Company | Fuel oils | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 0 | 0
Ameren Illinois Company | Fuel oils | Other 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 | 8 | 23
Ameren Illinois Company | Natural gas | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 5 | 11
Ameren Illinois Company | Natural gas | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 27 | 20
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 19 | 9
Ameren Illinois Company | Power | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 0 | 2
Ameren Illinois Company | Power | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | 0 | 4
Ameren Illinois Company | Power | Other current liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 10 | 2
Ameren Illinois Company | Power | Other deferred credits and liabilities |  |
Derivative [Line Items] |  |
Derivative Liability | 58 | 37
Ameren Illinois Company | Uranium | Other current assets |  |
Derivative [Line Items] |  |
Derivative assets | 0 | 0
Ameren Illinois Company | Uranium | Other assets |  |
Derivative [Line Items] |  |
Derivative assets | $ 0 | $ 0

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