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Other Commitments And Contingencies (Estimated Incident Expenses) (Details) - USD ($)
$ in Millions
 12 Months Ended 16 Months Ended 28 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2020
Estimated Incident Expenses [Line Items]      
Expenses Related to Third-Party Claims, Fines, Penalties, and settlements $ (5) $ 1,041 $ 1,036
Expenses Other Than Third Party Claims $ 22 $ 420 442
Maximum | Columbia Of Massachusetts      
Estimated Incident Expenses [Line Items]      
Expenses Related to Third-Party Claims, Fines, Penalties, and settlements     1,050
Expenses Other Than Third Party Claims     450
Minimum | Columbia Of Massachusetts      
Estimated Incident Expenses [Line Items]      
Expenses Related to Third-Party Claims, Fines, Penalties, and settlements     1,036
Expenses Other Than Third Party Claims     $ 445