SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Description of Business (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) state company $ / 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	$ 1.00
RLI Ins.
DESCRIPTION OF BUSINESS
Number of states in which entity operates	50
Mt. Hawley Insurance Company
DESCRIPTION OF BUSINESS
Number of states in which entity operates	50
Contractors Bonding and Insurance Company
DESCRIPTION OF BUSINESS
Number of states in which entity operates	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