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Basis of Presentation and Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Accounting Standards Update | us-gaap:AccountingStandardsUpdate201613Member |  |
Total shareholders' equity available to Arch | $ 12,451,997 | $ 11,497,371 | $ 11,158,096
Cumulative effect of an accounting change | Retained earnings |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Total shareholders' equity available to Arch |  | $ 22,500 |

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