Allowance for Expected Credit Losses - Contractholder receivables (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Credit Loss [Abstract]
Balance at beginning of period	$ 2,280	$ 2,016	$ 2,161	$ 1,814
Balance at end of period	2,287	2,078	2,287	2,078
Allowance for Expected Credit Losses
Balance at beginning of period	6	4	5	3
Change for provision of expected credit losses	0	0	1	1
Balance at end of period	$ 6	$ 4	$ 6	$ 4

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.”

No definition available.

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