Name:	rli_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
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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.

Name:	rli_AvailableForSaleSecuritiesGrossUnrealizedGainLossAccumulatedInInvestments
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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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Data Type:	xbrli:monetaryItemType
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Represents the gross unrealized losses on investments as of the balance sheet date.

No definition available.

Name:	rli_InvestmentsGrossUnrealizedLosses
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Data Type:	xbrli:monetaryItemType
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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
Balance Type:	na
Period Type:	duration

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Data Type:	xbrli:monetaryItemType
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- References

No definition available.

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- References

No definition available.

Name:	us-gaap_InvestmentsAbstract
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