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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Description of Business (Details)
 12 Months Ended
Dec. 31, 2015
state
company
DESCRIPTION OF BUSINESS  
Number of insurance companies through which the entity conducts operations | company 3
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