Name:	acgl_PercentageOfTotalAssetsAndLiabilitiesMeasuredAtFairValuePricedUsingNonBindingBrokerQuotes
Namespace Prefix:	acgl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Represents the aggregate of the assets and liabilities reported on the balance sheet at period end measured at fair value.

No definition available.

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

Represents the aggregate of the assets and liabilities reported on the balance sheet at period end measured at fair value which were priced using non-binding broker quotes.

No definition available.

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

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.

No definition available.

Name:	us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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- References

No definition available.

Name:	us-gaap_FairValueDisclosuresAbstract
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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

No definition available.

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_SeniorLongTermNotes
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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Name:	dei_LegalEntityAxis=acgl_WatfordHoldingsLtdMember
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Data Type:	na
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Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
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Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_InvestmentTypeAxis=acgl_FairValueOptionMember
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Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_OtherInvestmentsMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_InvestmentTypeAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
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