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General (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
1 Months Ended 0 Months Ended
Oct. 30, 2001
Mar. 20, 2014
Dec. 31, 2014
Dec. 31, 2013
Variable Interest Entity [Line Items]        
Proceeds from from common shares issued $ 763.2us-gaap_ProceedsFromIssuanceOfCommonStock      
Common shares, par value per share     $ 0.0033us-gaap_CommonStockParOrStatedValuePerShare $ 0.0033us-gaap_CommonStockParOrStatedValuePerShare
Watford Holdings Ltd | Variable Interest Entity, Primary Beneficiary        
Variable Interest Entity [Line Items]        
Percentage ownership   11.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ dei_LegalEntityAxis
= acgl_WatfordHoldingsLtdMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
   
Initial investment contribution amount   $ 100.0us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates
/ dei_LegalEntityAxis
= acgl_WatfordHoldingsLtdMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember