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Commitments and Contingencies - Concentrations of credit risk (Details) (USD $)
In Billions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Concentration Risk [Line Items]      
Net reinsurance recoverables 1.97acgl_ReinsuranceRecoverableNet 1.94acgl_ReinsuranceRecoverableNet  
Aon Corporation [Member] | Customer Concentration Risk [Member] | Gross Written Premiums [Member]      
Concentration Risk [Line Items]      
Concentration risk percentage 13.40%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= acgl_GrossWrittenPremiumsMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= acgl_AonCorporationMember		 16.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= acgl_GrossWrittenPremiumsMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= acgl_AonCorporationMember		 20.30%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= acgl_GrossWrittenPremiumsMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= acgl_AonCorporationMember
Marsh And McLennan Companies [Member] | Customer Concentration Risk [Member] | Gross Written Premiums [Member]      
Concentration Risk [Line Items]      
Concentration risk percentage 15.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= acgl_GrossWrittenPremiumsMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= acgl_MarshAndMclennanCompaniesMember		 17.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= acgl_GrossWrittenPremiumsMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= acgl_MarshAndMclennanCompaniesMember		 14.10%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= acgl_GrossWrittenPremiumsMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= acgl_MarshAndMclennanCompaniesMember