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Investments In Fixed Maturity Securities Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]    
Amortized Cost, Due in one year or less $ 811,966wrb_MarketableSecuritiesFixedMaturitiesWithinOneYearAmortizedCost  
Amortized Cost, Due after one year through five years 4,060,941wrb_MarketableSecuritiesFixedMaturitiesAfterOneThroughFiveYearsAmortizedCost  
Amortized Cost, Due after five years through ten years 3,612,480wrb_MarketableSecuritiesFixedMaturitiesAfterFiveThroughTenYearsAmortizedCost  
Amortized Cost, Due after ten years 2,449,041wrb_MarketableSecuritiesFixedMaturitiesAfterTenYearsAmortizedCost  
Amortized Cost, Mortgaged-backed securities 1,193,505wrb_MarketableSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost  
Total investments in fixed maturity securities, Amortized Value 12,127,933wrb_MarketableSecuritiesFixedMaturitiesAmortizedCost 12,223,264wrb_MarketableSecuritiesFixedMaturitiesAmortizedCost
Fair Value, Due in one year or less 814,139wrb_MarketableSecuritiesFixedMaturitiesWithinOneYearFairValue  
Fair Value, Due after one year through five years 4,247,879wrb_MarketableSecuritiesFixedMaturitiesAfterOneThroughFiveYearsFairValue  
Fair Value, Due after five years through ten years 3,816,097wrb_MarketableSecuritiesFixedMaturitiesAfterFiveThroughTenYearsFairValue  
Fair Value, Due after ten years 2,571,423wrb_MarketableSecuritiesFixedMaturitiesAfterTenYearsFairValue  
Fair Value, Mortgage-backed securities 1,223,815wrb_MarketableSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue  
Total investments in fixed maturity securities, Fair Value $ 12,673,353wrb_MarketableSecuritiesFixedMaturitiesFairValue $ 12,725,806wrb_MarketableSecuritiesFixedMaturitiesFairValue