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Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2020 | Dec. 31, 2019
Ceded Credit Risk [Line Items] |  |
Reinsurance recoverable on unpaid and paid losses and LAE | $ 4,640 | $ 4,350
Reinsurance recoverable | Credit concentration risk | AM Best A minus Or Better Rating |  |
Ceded Credit Risk [Line Items] |  |
Concentration risk percentage | 63.20% | 61.20%
Reinsurance recoverable | Credit concentration risk | Not Rated |  |
Ceded Credit Risk [Line Items] |  |
Concentration risk percentage | 36.60% | 38.80%
Reinsurance recoverable | Credit concentration risk | Not Rated | Reinsurance trusts or letters of credit |  |
Ceded Credit Risk [Line Items] |  |
Concentration risk percentage | 90.00% |
Stockholders' equity | Reinsurer concentration risk |  |
Ceded Credit Risk [Line Items] |  |
Concentration risk percentage | 1.80% | 1.70%

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount before valuation allowance recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References No definition available. + Details
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