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v3.21.2

Subsequent Events (Details) $ in Millions | Jul. 01, 2021 USD ($)
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Subsequent Event | Variable Interest Entity, Primary Beneficiary | Watford Holdings Ltd |
Subsequent Event [Line Items] |
Net gain on deconsolidation | $ 65

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- Definition Amount of gain (loss) from deconsolidation of a variable interest entity pursuant to a merger agreement, net. + References No definition available. + Details
Name: | acgl_DeconsolidationGainLossAmountNet
Namespace Prefix: | acgl_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_ConsolidatedEntitiesAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | dei_LegalEntityAxis=acgl_WatfordHoldingsLtdMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |