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Contingencies - Narrative (Details) - USD ($) $ in Millions |  | 3 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 24, 2015 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Reinsurance recoverables on asbestos and environmental reserves, net of allowance |  |  |  |  | $ 2,957 | $ 2,302 | $ 2,201 | $ 2,227
Property and Casualty Group |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Increase in asbestos and environmental reserves |  | $ 89 | $ 36 | $ 67 |  |  |  |
Liability for asbestos and environmental reserves |  |  |  |  | 528 |  |  |
Former railroad and manufacturing operations |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Increase in asbestos and environmental reserves |  | $ 24 | $ 5 | $ 12 |  |  |  |
American Premier and its subsidiaries |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Liability for environmental and other claims |  |  |  |  | 81 |  |  |
GAFRI |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Liability for environmental and other claims |  |  |  |  | 8 |  |  |
Run-off long-term care insurance business |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
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 |  |  |  |  | $ 125 |  |  |

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