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Fair Value - Rollforward of Level 3 assets and liabilities (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Level 3 assets rollforward    
Balance at beginning of year $ 549,990us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue $ 477,956us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Total gains or (losses) (realized/unrealized) - included in earnings 16,779us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 [1] 25,337us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 [1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income 252us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss (2,578)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
Purchases, issuances, sales and settlements    
Purchases 185,224us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases 300,067us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Issuances 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
Sales (68,209)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales (116,811)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Settlements (114,149)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements (133,981)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
Transfers in and/or out of Level 3 57,500us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
Balance at end of year 627,387us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue 549,990us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Contingent consideration liability    
Level 3 liabilities rollforward    
Balance at beginning of year 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember
Total gains or (losses) (realized/unrealized) - included in earnings (20,083)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember		 [1] 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember		 [1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember
Purchases, issuances, sales and settlements    
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember
Issuances (41,762)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember
Sales 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember
Settlements 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember
Transfers in and/or out of Level 3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember
Balance at end of year (61,845)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= acgl_ContingentConsiderationLiabilityMember
Available for sale | Asset backed securities    
Level 3 assets rollforward    
Balance at beginning of year 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Total gains or (losses) (realized/unrealized) - included in earnings 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 [1] 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 [1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Purchases, issuances, sales and settlements    
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Issuances 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Sales 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Settlements 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Transfers in and/or out of Level 3 57,500us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Balance at end of year 57,500us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Available for sale | Corporate bonds    
Level 3 assets rollforward    
Balance at beginning of year 2,045us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 98,404us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Total gains or (losses) (realized/unrealized) - included in earnings 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 [1] 4,679us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 [1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 (3,051)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Purchases, issuances, sales and settlements    
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Issuances 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Sales (2,045)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 (96,655)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Settlements 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 (1,332)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Transfers in and/or out of Level 3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Balance at end of year 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 2,045us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Available for sale | Other investments    
Level 3 assets rollforward    
Balance at beginning of year 170,420us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 184,202us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Total gains or (losses) (realized/unrealized) - included in earnings (39)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 [1] 8,915us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 [1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income (680)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 473us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Purchases, issuances, sales and settlements    
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 25,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Issuances 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
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= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Sales (66,164)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Settlements 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 (48,170)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Transfers in and/or out of Level 3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
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= us-gaap_AvailableforsaleSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Balance at end of year 103,537us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		 170,420us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Fair value option | Other investments    
Level 3 assets rollforward    
Balance at beginning of year 377,525us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember		 195,350us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember
Total gains or (losses) (realized/unrealized) - included in earnings 16,818us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember		 [1] 11,743us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember		 [1]
Total gains or (losses) (realized/unrealized) - included in other comprehensive income 932us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember
Purchases, issuances, sales and settlements    
Purchases 185,224us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember		 275,067us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember
Issuances 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember
Sales 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember		 (20,156)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember
Settlements (114,149)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember		 (84,479)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember
Transfers in and/or out of Level 3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember
Balance at end of year $ 466,350us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember		 $ 377,525us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_OtherInvestmentsMember
/ us-gaap_InvestmentTypeAxis
= acgl_FairValueOptionMember
[1] Gains or losses on corporate bonds were included in net realized gains (losses) while gains or losses on other investments were included in net realized gains (losses) or net investment income. Amounts related to the contingent consideration liability were included in net realized gains (losses).