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Variable Interest Entity and Noncontrolling Interests - Redeemable noncontrolling interest (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Redeemable Noncontrolling Interest [Line Items]      
Preferred shares, par value per share $ 0.01us-gaap_PreferredStockParOrStatedValuePerShare    
Amounts attributable to noncontrolling interests $ 13,095us-gaap_NetIncomeLossAttributableToNoncontrollingInterest [1] $ 0us-gaap_NetIncomeLossAttributableToNoncontrollingInterest [1] $ 0us-gaap_NetIncomeLossAttributableToNoncontrollingInterest [1]
Amounts attributable to noncontrolling interests (27,823)us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest 0us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest 0us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest
Dividends attributable to redeemable noncontrolling interests 14,728us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest 0us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest 0us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest
Watford Re      
Redeemable Noncontrolling Interest [Line Items]      
Preference shares, number of shares issued 9,065,200us-gaap_PreferredStockSharesIssued
/ dei_LegalEntityAxis
= acgl_WatfordReMember
   
Preferred shares, par value per share $ 0.01us-gaap_PreferredStockParOrStatedValuePerShare
/ dei_LegalEntityAxis
= acgl_WatfordReMember
   
Liquidation preference per share $ 25.00us-gaap_PreferredStockLiquidationPreference
/ dei_LegalEntityAxis
= acgl_WatfordReMember
   
Issue price per share $ 24.50us-gaap_SaleOfStockPricePerShare
/ dei_LegalEntityAxis
= acgl_WatfordReMember
   
Preferred shares, dividend rate (as a percent) 8.50%us-gaap_PreferredStockDividendRatePercentage
/ dei_LegalEntityAxis
= acgl_WatfordReMember
   
Other      
Redeemable Noncontrolling Interest [Line Items]      
Amounts attributable to noncontrolling interests (27,823)us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
0us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
0us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
Dividends attributable to redeemable noncontrolling interests 14,728us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
0us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
0us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
3 month US dollar LIBOR | Watford Re      
Redeemable Noncontrolling Interest [Line Items]      
Preferred shares, dividend rate (as a percent) 1.00%us-gaap_PreferredStockDividendRatePercentage
/ dei_LegalEntityAxis
= acgl_WatfordReMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
   
Redeemable preference shares | Watford Re      
Redeemable Noncontrolling Interest [Line Items]      
Noncontrolling interest, ownership percentage by noncontrolling owners 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ dei_LegalEntityAxis
= acgl_WatfordReMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RedeemablePreferredStockMember
   
Noncontrolling Interest [Member] | Watford Re      
Redeemable Noncontrolling Interest [Line Items]      
Amounts attributable to noncontrolling interests $ (27,823)us-gaap_NetIncomeLossAttributableToNonredeemableNoncontrollingInterest
/ dei_LegalEntityAxis
= acgl_WatfordReMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_NoncontrollingInterestMember
   
[1] See Note 4.