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Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions | Jun. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Derivative [Line Items] |  |
Gross derivative asset amount recognized on the balance sheet | $ 24 | $ 22
Gross derivative instruments not offset in the balance sheet | 7 | 7
Gross cash collateral received not offset in the balance sheet | 0 | 0
Net derivative asset | 17 | 15
Gross derivative liability amount recognized on the balance sheet | 141 | 168
Gross derivative instruments not offset in the balance sheet | 7 | 7
Gross cash collateral posted not offset in the balance sheet | 0 | 6
Net derivative liability | 134 | 155
Union Electric Company |  |
Derivative [Line Items] |  |
Gross derivative asset amount recognized on the balance sheet | 20 | 19
Gross derivative instruments not offset in the balance sheet | 5 | 6
Gross cash collateral received not offset in the balance sheet | 0 | 0
Net derivative asset | 15 | 13
Gross derivative liability amount recognized on the balance sheet | 19 | 25
Gross derivative instruments not offset in the balance sheet | 5 | 6
Gross cash collateral posted not offset in the balance sheet | 0 | 0
Net derivative liability | 14 | 19
Ameren Illinois Company |  |
Derivative [Line Items] |  |
Gross derivative asset amount recognized on the balance sheet | 4 | 3
Gross derivative instruments not offset in the balance sheet | 2 | 1
Gross cash collateral received not offset in the balance sheet | 0 | 0
Net derivative asset | 2 | 2
Gross derivative liability amount recognized on the balance sheet | 122 | 143
Gross derivative instruments not offset in the balance sheet | 2 | 1
Gross cash collateral posted not offset in the balance sheet | 0 | 6
Net derivative liability | $ 120 | $ 136

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