Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Assets measured at Fair Value
Available-for-sale fixed income	$ 3,536,484	$ 3,175,796
Equity securities	878,872	736,191
U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	399,593	515,635
U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	55,483	54,338
Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	12,377	7,871
Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	604,351	396,223
ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	563,754	410,248
Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,490,035	1,346,521
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	410,891	444,960
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	3,536,484	3,175,796
Equity securities	878,872	736,191
Total assets at fair value	4,415,356	3,911,987
Fair Value Measured on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at Fair Value
Equity securities	874,621	731,569
Total assets at fair value	874,621	731,569
Fair Value Measured on Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	3,439,225	3,085,293
Total assets at fair value	3,439,225	3,085,293
Fair Value Measured on Recurring Basis \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	97,259	90,503
Equity securities	4,251	4,622
Total assets at fair value	101,510	95,125
Fair Value Measured on Recurring Basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	399,593	515,635
Fair Value Measured on Recurring Basis \| U.S. government \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	399,593	515,635
Fair Value Measured on Recurring Basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	55,483	54,338
Fair Value Measured on Recurring Basis \| U.S. Agency \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	55,483	54,338
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	12,377	7,871
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	11,376	6,898
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	1,001	973
Fair Value Measured on Recurring Basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	604,351	396,223
Fair Value Measured on Recurring Basis \| Agency MBS \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	604,351	396,223
Fair Value Measured on Recurring Basis \| ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	563,754	410,248
Fair Value Measured on Recurring Basis \| ABS/CMBS/MBS \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	563,754	410,248
Fair Value Measured on Recurring Basis \| Corporate
Assets measured at Fair Value
Available-for-sale fixed income	1,490,035	1,346,521
Fair Value Measured on Recurring Basis \| Corporate \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	1,393,777	1,256,991
Fair Value Measured on Recurring Basis \| Corporate \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	96,258	89,530
Fair Value Measured on Recurring Basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	410,891	444,960
Fair Value Measured on Recurring Basis \| Municipal \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	$ 410,891	$ 444,960

Fair value portion of asset recognized for present right to economic benefit.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

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

- Definition

Amount excluding accrued interest, 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/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479130/326-30-45-1

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest
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), classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482736/825-10-45-1A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483467/210-10-45-1

+ 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

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=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_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:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: