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INVESTMENTS (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Fixed income securities in unrealized loss positions      
Unrealized Loss $ 5,095,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss   $ 7,119,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Unrealized losses as percentage of fixed income portfolio cost basis 0.30%rli_InvestmentContinuousUnrealizedLossPositionUnrealizedLossesAsPercentOfCost    
Other-than-temporary impairment (OTTI) losses recognized in other comprehensive earnings 0us-gaap_ImpairmentOfInvestments 0us-gaap_ImpairmentOfInvestments  
Debt securities      
Fixed income securities in unrealized loss positions      
Number of unrealized loss positions 154rli_InvestmentContinuousUnrealizedLossPositionNumber
/ us-gaap_InvestmentTypeAxis
= us-gaap_DebtSecuritiesMember
   
Number of securities in unrealized loss positions for 12 months or longer 33rli_InvestmentContinuousUnrealizedLossPositionTwelveMonthsOrLongerNumber
/ us-gaap_InvestmentTypeAxis
= us-gaap_DebtSecuritiesMember
   
Unrealized Loss 4,100,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_DebtSecuritiesMember
   
Common Stock      
Fixed income securities in unrealized loss positions      
Number of unrealized loss positions 4rli_InvestmentContinuousUnrealizedLossPositionNumber
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember
   
Number of securities in unrealized loss positions for 12 months or longer 0rli_InvestmentContinuousUnrealizedLossPositionTwelveMonthsOrLongerNumber
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember
   
Unrealized Loss 1,000,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember
   
Other-than-temporary impairment (OTTI) losses recognized in other comprehensive earnings $ 0us-gaap_ImpairmentOfInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember
$ 0us-gaap_ImpairmentOfInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_CommonStockMember