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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: