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Acquisitions and Sales of Businesses - Narrative (Details) - USD ($) shares in Millions, $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2017 | Jun. 30, 2020 | Jun. 10, 2019
Business Acquisition [Line Items] |  |  |  |  |  |  |
Income tax expense (benefit) | $ (30) | $ 34 | $ (63) | $ 171 |  |  |
Neon Capital Limited |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Estimated proceeds from divestiture of Neon | $ 6 |  |  |  |  |  |
Carrying value of assets and liabilities as a percentage of total assets and liabilities | 1.00% |  | 1.00% |  |  |  |
Income tax expense (benefit) | $ (73) |  |  |  |  |  |
Atlas Financial Holdings - Paratransit Business |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Gross written premiums eligible for renewal |  |  |  |  | $ 110 |  |
Business acquisition, price as a percent of gross written premiums |  |  |  |  |  |  | 15.00%
Warrant term to acquire shares |  |  |  |  |  |  | 5 years
Number of shares able to be acquired with the warrant |  |  |  |  |  |  | 2.4
Business acquisition, percentage of entity acquired by warrants |  |  |  |  |  |  | 19.90%
Neon Capital Limited | Neon Capital Limited Executives |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Percentage of indirect noncontrolling interest acquired |  |  |  |  |  | 100.00% |
Equity securities | Public company warrants | Atlas Financial Holdings - Paratransit Business |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Fair value of warrant on date received |  |  |  |  |  |  | $ 1

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