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Schedule IV - Reinsurance (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]                      
Gross Amount                 $ 3,100,946us-gaap_DirectPremiumsWritten $ 2,754,582us-gaap_DirectPremiumsWritten $ 2,673,864us-gaap_DirectPremiumsWritten
Ceded to other Companies                 (948,678)us-gaap_CededPremiumsWritten (845,256)us-gaap_CededPremiumsWritten (816,926)us-gaap_CededPremiumsWritten
Assumed From Other Companies                 1,739,670us-gaap_AssumedPremiumsWritten 1,442,041us-gaap_AssumedPremiumsWritten 1,195,297us-gaap_AssumedPremiumsWritten
Net Amount 895,481us-gaap_PremiumsWrittenNet 959,539us-gaap_PremiumsWrittenNet 971,928us-gaap_PremiumsWrittenNet 1,064,990us-gaap_PremiumsWrittenNet 748,921us-gaap_PremiumsWrittenNet 839,135us-gaap_PremiumsWrittenNet 810,535us-gaap_PremiumsWrittenNet 952,776us-gaap_PremiumsWrittenNet 3,891,938us-gaap_PremiumsWrittenNet 3,351,367us-gaap_PremiumsWrittenNet 3,052,235us-gaap_PremiumsWrittenNet
Percentage of Amount Assumed to Net                 44.70%acgl_PremiumsWrittenPercentageAssumedToNet 43.00%acgl_PremiumsWrittenPercentageAssumedToNet 39.20%acgl_PremiumsWrittenPercentageAssumedToNet
Insurance                      
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]                      
Gross Amount                 2,974,996us-gaap_DirectPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 2,682,446us-gaap_DirectPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 2,562,788us-gaap_DirectPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember
Ceded to other Companies                 (862,015)us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 [1] (763,713)us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 [1] (768,625)us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 [1]
Assumed From Other Companies                 33,673us-gaap_AssumedPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 [1] 30,063us-gaap_AssumedPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 [1] 31,171us-gaap_AssumedPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 [1]
Net Amount                 2,146,654us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 1,948,796us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 1,825,334us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember
Percentage of Amount Assumed to Net                 1.60%acgl_PremiumsWrittenPercentageAssumedToNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 1.50%acgl_PremiumsWrittenPercentageAssumedToNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember		 1.70%acgl_PremiumsWrittenPercentageAssumedToNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_InsuranceSegmentMember
Reinsurance                      
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]                      
Gross Amount                 22,405us-gaap_DirectPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 72,136us-gaap_DirectPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 111,076us-gaap_DirectPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember
Ceded to other Companies                 (261,255)us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 [1] (86,620)us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 [1] (55,099)us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 [1]
Assumed From Other Companies                 1,504,841us-gaap_AssumedPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 [1] 1,327,485us-gaap_AssumedPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 [1] 1,102,813us-gaap_AssumedPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 [1]
Net Amount                 1,265,991us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 1,313,001us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 1,158,790us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember
Percentage of Amount Assumed to Net                 118.90%acgl_PremiumsWrittenPercentageAssumedToNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 101.10%acgl_PremiumsWrittenPercentageAssumedToNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember		 95.20%acgl_PremiumsWrittenPercentageAssumedToNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_ReinsuranceSegmentMember
Mortgage                      
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]                      
Gross Amount                 103,545us-gaap_DirectPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 0us-gaap_DirectPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 0us-gaap_DirectPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember
Ceded to other Companies                 (22,519)us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 [1] 0us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 [1] 0us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 [1]
Assumed From Other Companies                 123,811us-gaap_AssumedPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 [1] 89,570us-gaap_AssumedPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 [1] 68,111us-gaap_AssumedPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 [1]
Net Amount                 204,837us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 89,570us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 68,111us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember
Percentage of Amount Assumed to Net                 60.40%acgl_PremiumsWrittenPercentageAssumedToNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 100.00%acgl_PremiumsWrittenPercentageAssumedToNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember		 100.00%acgl_PremiumsWrittenPercentageAssumedToNet
/ us-gaap_StatementBusinessSegmentsAxis
= acgl_MortgageSegmentMember
Other                      
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]                      
Gross Amount                 0us-gaap_DirectPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		    
Ceded to other Companies                 (14,171)us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 [1] 0us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 0us-gaap_CededPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
Assumed From Other Companies                 288,627us-gaap_AssumedPremiumsWritten
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 [1]    
Net Amount                 $ 274,456us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 $ 0us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 $ 0us-gaap_PremiumsWrittenNet
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
Percentage of Amount Assumed to Net                 105.20%acgl_PremiumsWrittenPercentageAssumedToNet
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		    
[1] Certain amounts included in the gross premiums written of each segment are related to intersegment transactions and are included in the gross premiums written of each segment. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.