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Income Taxes - Statutory tax rate by jurisdiction (Details)
 12 Months Ended
Dec. 31, 2014
Bermuda  
Income Tax Disclosure [Line Items]  
Statutory tax rate (percentage) 0.00%acgl_StatutoryTaxRatePercentage
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
United States  
Income Tax Disclosure [Line Items]  
Statutory tax rate (percentage) 35.00%acgl_StatutoryTaxRatePercentage
/ us-gaap_IncomeTaxAuthorityAxis
= country_US
United Kingdom  
Income Tax Disclosure [Line Items]  
Statutory tax rate (percentage) 21.50%acgl_StatutoryTaxRatePercentage
/ us-gaap_IncomeTaxAuthorityAxis
= country_GB
Ireland  
Income Tax Disclosure [Line Items]  
Statutory tax rate (percentage) 12.50%acgl_StatutoryTaxRatePercentage
/ us-gaap_IncomeTaxAuthorityAxis
= country_IE
Denmark  
Income Tax Disclosure [Line Items]  
Statutory tax rate (percentage) 24.50%acgl_StatutoryTaxRatePercentage
/ us-gaap_IncomeTaxAuthorityAxis
= country_DK
Canada  
Income Tax Disclosure [Line Items]  
Statutory tax rate (percentage) 24.00%acgl_StatutoryTaxRatePercentage
/ us-gaap_IncomeTaxAuthorityAxis
= country_CA
Gibraltar  
Income Tax Disclosure [Line Items]  
Statutory tax rate (percentage) 10.00%acgl_StatutoryTaxRatePercentage
/ us-gaap_IncomeTaxAuthorityAxis
= country_GI
Netherlands  
Income Tax Disclosure [Line Items]  
Statutory tax rate (percentage) 23.90%acgl_StatutoryTaxRatePercentage
/ us-gaap_IncomeTaxAuthorityAxis
= country_NL