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Contingencies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Contingencies - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
Reinsurance recoverables on asbestos and environmental reserves, net of allowance			$ 3,419	$ 3,117	$ 3,024	$ 2,942
Property and Casualty Group
Loss Contingencies [Line Items]
Increase in asbestos and environmental reserves	$ 47	$ 18
Liability for asbestos and environmental reserves			555
Former railroad and manufacturing operations
Loss Contingencies [Line Items]
Increase in asbestos and environmental reserves	$ 21	$ 11
American Premier and its subsidiaries
Loss Contingencies [Line Items]
Liability for environmental and other claims			87
GAFRI
Loss Contingencies [Line Items]
Liability for environmental and other claims			7
A&E reserves \| Property and Casualty Group
Loss Contingencies [Line Items]
Reinsurance recoverables on asbestos and environmental reserves, net of allowance			$ 147