INVESTMENTS - Assets Measured At Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets measured at Fair Value
Available-for-sale fixed income	$ 1,760,515	$ 1,672,239
Equity securities, at fair value (cost - $220,373 in 2018 and $182,002 in 2017)	340,483	400,492
Transfers in (out of) level 1	0	0
Transfers in (out of) level 2	0	0
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	200,229	91,689
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	31,904	18,778
Non-U.S. govt. & agency
Assets measured at Fair Value
Available-for-sale fixed income	7,639	7,588
Mortgage-backed
Assets measured at Fair Value
Available-for-sale fixed income	395,253	328,471
ABS/CMBS
Assets measured at Fair Value
Available-for-sale fixed income	136,723	70,526
Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	668,679	519,022
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	320,088	636,165
Fair value measured on recurring basis
Assets measured at Fair Value
Available-for-sale fixed income	1,760,515	1,672,239
Total assets at fair value	2,100,998	2,072,731
Fair value measured on recurring basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	200,229	91,689
Fair value measured on recurring basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	31,904	18,778
Fair value measured on recurring basis \| Non-U.S. govt. & agency
Assets measured at Fair Value
Available-for-sale fixed income	7,639	7,588
Fair value measured on recurring basis \| Mortgage-backed
Assets measured at Fair Value
Available-for-sale fixed income	395,253	328,471
Fair value measured on recurring basis \| ABS/CMBS
Assets measured at Fair Value
Available-for-sale fixed income	136,723	70,526
Fair value measured on recurring basis \| Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	668,679	519,022
Fair value measured on recurring basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	320,088	636,165
Fair value measured on recurring basis \| Equity securities
Assets measured at Fair Value
Equity securities, at fair value (cost - $220,373 in 2018 and $182,002 in 2017)	340,483	400,492
Fair value measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at Fair Value
Total assets at fair value	339,985	400,492
Fair value measured on recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Equity securities
Assets measured at Fair Value
Equity securities, at fair value (cost - $220,373 in 2018 and $182,002 in 2017)	339,985	400,492
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	1,760,515	1,672,239
Total assets at fair value	1,761,013	1,672,239
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	200,229	91,689
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	31,904	18,778
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Non-U.S. govt. & agency
Assets measured at Fair Value
Available-for-sale fixed income	7,639	7,588
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Mortgage-backed
Assets measured at Fair Value
Available-for-sale fixed income	395,253	328,471
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| ABS/CMBS
Assets measured at Fair Value
Available-for-sale fixed income	136,723	70,526
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	668,679	519,022
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	320,088	$ 636,165
Fair value measured on recurring basis \| Significant Other Observable Inputs (Level 2) \| Equity securities
Assets measured at Fair Value
Equity securities, at fair value (cost - $220,373 in 2018 and $182,002 in 2017)	$ 498

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

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

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

Name:	us-gaap_EquitySecuritiesFvNi
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=rli_AssetBackedSecuritiesAndCommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: