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Segments of Operations - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) segment | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Number of segments | segment |  |  |  |  | 3 |  |
Revenue derived from sales of property and casualty insurance outside of the United States |  |  |  |  | 7.00% | 5.00% | 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
Property and casualty insurance | Other lines |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves | $ 18 |  | $ 89 |  | $ 18 | $ 89 | 36
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
Other |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Special charges to increase asbestos and environmental reserves |  |  |  |  | $ 9 | 24 | $ 5
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 |

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- Definition Favorable prior year reserve development of the Company's liability for losses and loss adjustment expenses related to certain lines of business within subsidiaries including exited and ongoing lines of business in connection with a reinsurance to close agreement. + References No definition available. + Details
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