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Allowance for Expected Credit Losses - Contractholder receivables (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2021	Dec. 31, 2020
Credit Loss [Abstract]
Balance at beginning of period		$ 1,986,924	$ 2,119,460
Balance at end of period		1,919,655	1,986,924
Allowance for Expected Credit Losses
Balance at beginning of period		8,638	0
Cumulative effect of accounting change	[1]		6,663
Change for provision of expected credit losses		(2,785)	1,975
Balance at end of period		$ 5,853	$ 8,638
[1] Adoption of ASU 2016-13, “Financial Instruments - Credit Losses (Topic 326)”

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- Definition

Certain insurance policies written by the Company’s insurance operations feature large deductibles, primarily in its construction and national accounts lines of business. Under such contracts, the Company is obligated to pay the claimant for the full amount of the claim. The Company is subsequently reimbursed by the policyholder for the deductible amount. These amounts are included on a gross basis in the consolidated balance sheet in contractholder payables and contractholder receivables, respectively. In the event that the Company is unable to collect from the policyholder, the Company would be liable for such defaulted amounts. Collateral, primarily in the form of letters of credit, cash and trusts, is obtained from the policyholder to mitigate the Company’s credit risk. In the instances where the Company receives collateral in the form of cash, the Company reflects it in “Collateral held for insured obligations.”

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No definition available.

+ Details

Name:	acgl_ContractholderDeductiblesReceivable
Namespace Prefix:	acgl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- Definition

Amount of expense (reversal of expense) for expected credit loss on contractholder receivable from change in methodology.

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No definition available.

+ Details

Name:	acgl_ContractholderReceivableChangeinMethodCreditLossExpenseReversal
Namespace Prefix:	acgl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- Definition

Amount of allowance for credit loss on contractholder receivables.

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No definition available.

+ Details

Name:	acgl_ContractholderReceivablesAllowanceForCreditLoss
Namespace Prefix:	acgl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

Amount of expense (reversal of expense) for expected credit loss on contractholder receivables.

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No definition available.

+ Details

Name:	acgl_ContractholderReceivablesCreditLossExpenseReversal
Namespace Prefix:	acgl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

+ Details

Name:	us-gaap_ContractWithCustomerAssetAllowanceForCreditLossRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- References

No definition available.

+ Details

Name:	us-gaap_CreditLossAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration