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v3.21.2

Acquisitions (Details) - USD ($) $ in Millions |  | 3 Months Ended
--- | --- | ---
Aug. 31, 2021 | Sep. 30, 2021
Business Acquisition [Line Items] |  |
Increase in goodwill and intangible assets as a result of acquisitions during the period |  | $ 337.4
WLMI |  |
Business Acquisition [Line Items] |  |
Term of exclusivity agreement | 10 years |

X
- Definition The length of period that the acquiree will have exclusivity for new mortgage originations with its counterparty. + References No definition available. + Details
Name: | acgl_BusinessAcquisitionExclusivityAgreementTerm
Namespace Prefix: | acgl_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of increase (decrease) of goodwill and intangible assets arising from business combinations. + References No definition available. + Details
Name: | acgl_IntangibleAssetsIncludingGoodwillPeriodIncreaseDecrease
Namespace Prefix: | acgl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_BusinessAcquisitionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=acgl_WLMIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |