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Acquisitions and Sale of Businesses - Narrative on acquisitions of subsidiaries (Details) - USD ($) shares in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Nov. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jun. 10, 2019
Business Acquisitions [Line Items] |  |  |  |  |  |
Payments to acquire subsidiaries |  |  | $ 0 | $ 36 | $ 0 |
Goodwill acquired during period |  | $ 8 |  | $ 8 |  |
Atlas Financial Holdings - Paratransit Business |  |  |  |  |  |
Business Acquisitions [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%
ABA Insurance Services |  |  |  |  |  |
Business Acquisitions [Line Items] |  |  |  |  |  |
Payments to acquire subsidiaries | $ 30 |  |  |  |  |
Contingent consideration |  |  | $ 3 |  |  |
Contingent consideration term |  |  |  | 4 years |  |
Finite-lived intangible assets acquired | 25 |  |  |  |  |
Finite-lived intangible asset useful life |  |  |  | 9 years |  |
Goodwill acquired during period | $ 8 |  |  |  |  |
Equity securities | Warrant | Maximum | Atlas Financial Holdings - Paratransit Business |  |  |  |  |  |
Business Acquisitions [Line Items] |  |  |  |  |  |
Fair value of asset on date received |  |  |  |  |  | $ 1

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