Amount transferred from common stock to paid-in-capital as a result of a decrease in par value.

No definition available.

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- Definition

Represents the number of insurance companies through which the entity conducts operations.

+ References

No definition available.

+ Details

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- Definition

Amount reclassified out of common stock due to change in par value.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount reclassified into paid-in capital due to change in par value.

+ References

No definition available.

+ Details

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Namespace Prefix:	rli_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Face amount or stated value per share of common stock.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Period Type:	instant

- Definition

The number of states the entity operates in as of the balance sheet date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
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- 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

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- Details

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Namespace Prefix:
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- Details

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Namespace Prefix:
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- Details

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Description of Business (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019 company state $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 $ / shares
DESCRIPTION OF BUSINESS
Number of insurance companies through which the entity conducts operations \| company	3
Common Stock, Par or Stated Value Per Share \| $ / shares	$ 0.01	$ 0.01	$ 1.00
Reclassification From Common Stock		$ 66,400
Reclassification Into Paid In Capital		66,400
RLI Ins.
DESCRIPTION OF BUSINESS
Number of states in which entity operates \| state	50
Mt. Hawley Insurance Company
DESCRIPTION OF BUSINESS
Number of states in which entity operates \| state	50
Contractors Bonding and Insurance Company
DESCRIPTION OF BUSINESS
Number of states in which entity operates \| state	50
Common stock
DESCRIPTION OF BUSINESS
Reclassification due to change in par value		(66,409)
Paid-in Capital
DESCRIPTION OF BUSINESS
Reclassification due to change in par value		$ 66,409