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Insurance - Narrative (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Jun. 30, 2017 | Sep. 30, 2017 | Sep. 30, 2016 | Jun. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Catastrophe bonds |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |
Catastrophe reinsurance coverage, excess retention percentage | 95.00% |  |  |  |  |
Catastrophe reinsurance coverage, excess retention amount | $ 200 |  |  |  |  |
Catastrophe reinsurance coverage, retention amount | $ 100 |  |  |  |  |
Catastrophe reinsurance coverage, annual cost |  |  |  |  | $ 11 |
Property and Casualty Insurance | Other lines |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves |  | $ 89 | $ 36 |  | $ 89 | $ 36
Special charges related to the exit of certain lines of business within subsidiaries |  |  |  | $ 65 |  |
Property and Casualty Insurance | Increased insurance loss reserves related to medical malpractice and general liability lines | Other lines |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |
Special charges related to the exit of certain lines of business within subsidiaries |  |  |  |  |  | $ 57

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