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Contingencies - Narrative (Details) - USD ($) $ in Millions |  | 3 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 24, 2015 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Reinsurance recoverables on asbestos and environmental reserves, net of allowance |  |  |  |  | $ 3,117 | $ 3,024 | $ 2,942 | $ 2,957
Property and Casualty Group |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Increase in asbestos and environmental reserves |  | $ 47 | $ 18 | $ 18 |  |  |  |
Liability for asbestos and environmental reserves |  |  |  |  | 572 |  |  |
Former railroad and manufacturing operations |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Increase in asbestos and environmental reserves |  | $ 21 | $ 11 | $ 9 |  |  |  |
American Premier and its subsidiaries |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Liability for environmental and other claims |  |  |  |  | 94 |  |  |
GAFRI |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Liability for environmental and other claims |  |  |  |  | 7 |  |  |
Run-off long-term care and life |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Contingent capital support agreement amount | $ 35 |  |  |  |  |  |  |
Contingent capital support agreement term | 5 years |  |  |  |  |  |  |
A&E reserves | Property and Casualty Group |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Reinsurance recoverables on asbestos and environmental reserves, net of allowance |  |  |  |  | $ 150 |  |  |

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