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Equity Method Investments Time Lag For Reporting Low End of Range

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The rate of collateral required, expressed as a percentage of the fair value of loaned securities.

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[Line Items] for Significant Accounting Policies [Table]

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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Ratio applied to the conversion of stock split, for example but not limited to, one share converted to two or two shares converted to one.

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Significant Accounting Policies (Details) $ in Thousands	1 Months Ended	12 Months Ended
Significant Accounting Policies (Details) $ in Thousands	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