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Investment Information - Restricted Assets (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Restricted Assets [Line Items]    
Restricted assets $ 5,519,089acgl_RestrictedAssets $ 5,302,042acgl_RestrictedAssets
Collateral or guarantees - affiliated transactions    
Restricted Assets [Line Items]    
Restricted assets 4,138,527acgl_RestrictedAssets
/ acgl_RestrictedAssetsRestrictionTypeAxis
= acgl_CollateralOrGuaranteesAffiliatedTransactionsMember		 4,060,533acgl_RestrictedAssets
/ acgl_RestrictedAssetsRestrictionTypeAxis
= acgl_CollateralOrGuaranteesAffiliatedTransactionsMember
Collateral or guarantees - third party agreements    
Restricted Assets [Line Items]    
Restricted assets 970,120acgl_RestrictedAssets
/ acgl_RestrictedAssetsRestrictionTypeAxis
= acgl_CollateralOrGuaranteesThirdPartyAgreementsMember		 856,890acgl_RestrictedAssets
/ acgl_RestrictedAssetsRestrictionTypeAxis
= acgl_CollateralOrGuaranteesThirdPartyAgreementsMember
Deposits with US regulatory authorities    
Restricted Assets [Line Items]    
Restricted assets 337,981acgl_RestrictedAssets
/ acgl_RestrictedAssetsRestrictionTypeAxis
= acgl_DepositsWithUsRegulatoryAuthoritiesMember		 302,809acgl_RestrictedAssets
/ acgl_RestrictedAssetsRestrictionTypeAxis
= acgl_DepositsWithUsRegulatoryAuthoritiesMember
Deposits with non-US regulatory authorities    
Restricted Assets [Line Items]    
Restricted assets 0acgl_RestrictedAssets
/ acgl_RestrictedAssetsRestrictionTypeAxis
= acgl_DepositsWithNonUsRegulatoryAuthoritiesMember		 6,546acgl_RestrictedAssets
/ acgl_RestrictedAssetsRestrictionTypeAxis
= acgl_DepositsWithNonUsRegulatoryAuthoritiesMember
Trust funds    
Restricted Assets [Line Items]    
Restricted assets $ 72,461acgl_RestrictedAssets
/ acgl_RestrictedAssetsRestrictionTypeAxis
= acgl_TrustFundsMember		 $ 75,264acgl_RestrictedAssets
/ acgl_RestrictedAssetsRestrictionTypeAxis
= acgl_TrustFundsMember