SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Investments (Details)	Dec. 31, 2017 item category
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Number of categories for debt and equity securities \| category	3
Number of investments in low income housing tax credit partnerships \| item	3

Represents the number of categories into which investments in debt and equity securities are classified.

No definition available.

Name:	rli_InvestmentsInDebtAndEquitySecuritiesNumberOfCategories
Namespace Prefix:	rli_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant