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Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2022 | Jun. 30, 2021 | Jun. 30, 2022 | Jun. 30, 2021
Pension Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | $ (2) | $ 18 | $ (3) | $ 31
Pension Plan | Union Electric Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (1) | 9 | (2) | 15
Pension Plan | Ameren Illinois Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | 0 | 9 | 1 | 17
Pension Plan | Other Affiliated Entities And Intercompany Eliminations |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (1) | 0 | (2) | (1)
Other Postretirement Benefit Plan, Defined Benefit |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (14) | (9) | (27) | (17)
Other Postretirement Benefit Plan, Defined Benefit | Union Electric Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (4) | (1) | (7) | (2)
Other Postretirement Benefit Plan, Defined Benefit | Ameren Illinois Company |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | (10) | (8) | (20) | (15)
Other Postretirement Benefit Plan, Defined Benefit | Other Affiliated Entities And Intercompany Eliminations |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Net periodic benefit cost | $ 0 | $ 0 | $ 0 | $ 0

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