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Segments of Operations - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Dec. 31, 2017 USD ($) segment | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Number of segments | segment |  |  |  |  |  |  | 4 |  |
Revenue derived from sales of property and casualty insurance outside of the United States |  |  |  |  |  |  | 5.00% | 4.00% | 4.00%
Property and casualty insurance | Real estate investment |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Pretax income from the sale of real estate properties included in other income |  |  |  | $ 13 |  | $ 32 |  | $ 32 | $ 66
Property and casualty insurance | Other lines |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves |  | $ 89 |  |  | $ 36 |  | $ 89 | 36 | 67
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement |  |  |  |  |  |  | 18 |  |
Special charges related to the exit of certain lines of business within subsidiaries |  |  |  |  |  | $ 65 |  | 65 |
Other |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves |  |  |  |  |  |  | 24 | $ 5 | 12
Make-whole premium, settlement of interest rate swap and write off of deferred debt issuance cost included in the loss on the retirement of debt | $ 40 |  |  |  |  |  | $ 51 |  |
Write off of deferred debt issuance cost due to the retirement of debt |  | $ 4 | $ 7 |  |  |  |  |  | $ 4

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