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Summary of Significant Accounting Policies - Pro forma information Premium receivable reinsurance and Acquisition costs (Details) $ in Millions |  |  | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jun. 08, 2020 USD ($) | Jan. 01, 2019 USD ($) item shares | Sep. 30, 2020 USD ($) segment | Dec. 31, 2019 USD ($) segment shares | Dec. 31, 2018 USD ($) shares | Dec. 31, 2017 employee
Proceeds from release of shares upon repayment |  |  | $ 1.3 |  |  |
Number of reportable segments | segment |  |  | 1 | 1 |  |
Premium receivable, allowance for doubtful accounts |  |  | $ 0.4 | $ 0.2 | $ 0.0 |
Allowance for uncollectible reinsurance |  |  |  | 0.0 |  |
Amortization of deferred acquisition costs |  |  |  | $ 2.1 | 0.7 |
Options to purchase common stock | Executive |  |  |  |  |  |
Proceeds from release of shares upon repayment | $ 1.3 | $ 1.3 |  |  | $ 1.2 |
Subscriptions receivable, number of executive employees |  | 2 |  |  |  | 2
Restricted Stock | Executive |  |  |  |  |  |
Share Based Compensation Arrangements By Share Based Payments In Purchase Agreements | shares |  | 513,537 |  | 513,537 | 619,024 |

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