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Net amount of the difference between the fair value and market value on investments in available-for-sale securities.

No definition available.

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Amount of accumulated unrealized loss before deducting unrealized gain on investments in available-for-sale securities impacting investments.

No definition available.

Name:	rli_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Namespace Prefix:	rli_
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Represents the percentage of securities of portfolio rated as AA or better, which are categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	rli_AvailableForSaleSecuritiesPercentageOfSecuritiesRatedAsAAOrBetter
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Represents the percentage of securities of portfolio rated as A or better, which are categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	rli_AvailableForSaleSecuritiesPercentageOfSecuritiesRatedAsAOrBetter
Namespace Prefix:	rli_
Data Type:	num:percentItemType
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Represents percentage of the municipal fixed income securities in the investment portfolio that are general obligations of state and local governments.

No definition available.

Name:	rli_GeneralObligationsToStateAndLocalGovernmentsPercentage
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(Increase) decrease in amount of accumulated unrealized loss before deducting unrealized gain on investment in available-for-sale securities impacting investments during the period.

No definition available.

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Represents percentage of the municipal fixed income securities in the investment portfolio that are revenue based.

No definition available.

Name:	rli_RevenueBasedObligationsPercentage
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Data Type:	num:percentItemType
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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- References

No definition available.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Period Type:	instant

- References

No definition available.

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Name:	us-gaap_InvestmentTypeAxis=rli_AssetBackedSecuritiesAndCommercialMortgageBackedSecuritiesMember
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