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Segments of Operations - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2019 USD ($) segment | Dec. 31, 2018 USD ($) | Dec. 31, 2017 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% | 7.00% | 5.00%
Property and casualty insurance | Other lines |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Percent of foreign sales attributable to Neon |  |  |  |  | 66.67% |  |
Special A&E charges |  | $ 18 | $ 18 |  | $ 18 | $ 18 | $ 89
Special charges related to the exit of certain lines of business within subsidiaries | $ 76 |  |  |  |  |  |
Other |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Loss on the retirement of debt |  |  |  |  | 5 |  | 51
Special charges to increase asbestos and environmental reserves |  |  |  |  | 11 | 9 | 24
Special A&E charges |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Special A&E charges |  |  |  |  | 18 | $ 18 | 89
Special A&E charges | Property and casualty insurance | Other lines |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Special A&E charges |  |  |  |  | 18 |  | 89
Neon Capital Limited | Property and casualty insurance |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement |  |  |  | $ 42 |  |  |
Neon Capital Limited | Property and casualty insurance | Other lines |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |
Special charges related to the exit of certain lines of business within subsidiaries |  |  |  |  | $ 76 |  |
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement |  |  |  | $ 18 |  |  | $ 18

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