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Segments of Operations - Narrative (Details) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) segment | Sep. 30, 2016 USD ($)
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Number of segments | segment |  |  |  |  |  | 4 |
Property and Casualty Insurance | Real Estate Investment |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Pretax income from the sale of real estate properties |  |  | $ 13 |  | $ 32 |  |
Other |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves | $ 24 |  |  | $ 5 |  |  |
Write off of deferred debt issuance cost due to the retirement of debt | 4 | $ 7 |  |  |  |  |
Other lines | Property and Casualty Insurance |  |  |  |  |  |  |
Segment Reporting Information [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 |  |

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