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Significant Accounting Policies (Details) $ in Thousands | 1 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
May 31, 2018 | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items] |  |  |  |  |
Share split conversion ratio | 3 |  |  |  |
Premium revenue recognition period |  | 12 months |  |  |
Securities lending rate of collateral required (percentage) |  | 102.00% |  |  |
Equity method investments, time lag for reporting - low end of range |  | one |  |  |
Equity method investments, time lag for reporting - high end of range |  | three |  |  |
Vesting period, share-based awards |  | 3 years |  |  |
Requisite service period, share-based awards |  | 3 years |  |  |
Total shareholders' equity available to Arch |  | $ (13,105,886) | $ (11,497,371) | $ (9,439,827) |
Losses occurring |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Premium revenue recognition period |  | 12 months |  |  |
Risks attaching |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Premium revenue recognition period |  | 24 |  |  |
Retained earnings |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Total shareholders' equity available to Arch |  | $ (12,362,463) | (11,021,006) | (9,426,299) | $ (8,562,889)
Retained earnings | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Total shareholders' equity available to Arch |  |  | $ 22,500 | $ 0 |

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