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Variable Interest Entity and Noncontrolling Interests - Non-redeemable noncontrolling interest (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Noncontrolling Interest [Line Items]      
Non-redeemable noncontrolling interest, beginning of year $ 0us-gaap_MinorityInterest [1] $ 0us-gaap_MinorityInterest [1]  
Amounts attributable to noncontrolling interests (27,823)us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest 0us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest 0us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest
Non-redeemable noncontrolling interest, end of year 769,081us-gaap_MinorityInterest [1] 0us-gaap_MinorityInterest [1] 0us-gaap_MinorityInterest [1]
Watford Re | Noncontrolling Interest [Member]      
Noncontrolling Interest [Line Items]      
Non-redeemable noncontrolling interest, beginning of year 0us-gaap_MinorityInterest
/ dei_LegalEntityAxis
= acgl_WatfordReMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_NoncontrollingInterestMember
   
Sale of shares to noncontrolling interests 796,904us-gaap_NoncontrollingInterestIncreaseFromSubsidiaryEquityIssuance
/ dei_LegalEntityAxis
= acgl_WatfordReMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_NoncontrollingInterestMember
   
Amounts attributable to noncontrolling interests (27,823)us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest
/ dei_LegalEntityAxis
= acgl_WatfordReMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_NoncontrollingInterestMember
   
Non-redeemable noncontrolling interest, end of year $ 769,081us-gaap_MinorityInterest
/ dei_LegalEntityAxis
= acgl_WatfordReMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_NoncontrollingInterestMember
   
Common shares | Watford Re | Noncontrolling Interest [Member]      
Noncontrolling Interest [Line Items]      
Noncontrolling interest, ownership percentage by noncontrolling owners 89.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ dei_LegalEntityAxis
= acgl_WatfordReMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_NoncontrollingInterestMember
   
[1] See Note 4.