Represents the additional amount of the catastrophe reinsurance protection.

No definition available.

Name:	rli_CatastropheReinsuranceAdditionalAmount
Namespace Prefix:	rli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of catastrophe exposure covered by catastrophe reinsurance protection.

+ References

No definition available.

+ Details

Name:	rli_CatastropheReinsuranceAmount
Namespace Prefix:	rli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of first-dollar liability retained by the entity on catastrophe exposures under reinsurance protection.

+ References

No definition available.

+ Details

Name:	rli_CatastropheReinsuranceFirstDollarRetention
Namespace Prefix:	rli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the total amount of catastrophe exposure covered by catastrophe reinsurance protection.

+ References

No definition available.

+ Details

Name:	rli_CatastropheReinsuranceTotalAmount
Namespace Prefix:	rli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

- Details

Name:	us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=rli_EarthquakeInCaliforniaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=rli_EarthquakeInNonCaliforniaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=rli_OtherPerilsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Catastrophe Exposures (Details) - USD ($) $ in Millions		2 Months Ended	10 Months Ended
	Mar. 01, 2020	Feb. 29, 2020	Dec. 31, 2020
Catastrophe Exposures
First-dollar retention		$ 25	$ 25
Additional catastrophe reinsurance	$ 100
California earthquake
Catastrophe Exposures
Catastrophe reinsurance		400
First-dollar retention		25
Total catastrophe reinsurance			500
Non-California earthquake
Catastrophe Exposures
Catastrophe reinsurance		425
First-dollar retention		25
Total catastrophe reinsurance			525
Other perils
Catastrophe Exposures
Catastrophe reinsurance		275
First-dollar retention		$ 25
Total catastrophe reinsurance			$ 375