STATUTORY INFORMATION AND DIVIDEND RESTRICTIONS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) company	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Statutory information and dividend restrictions
Number of insurance companies | company	3
Equity	$ 823,469	$ 845,062	$ 828,966	$ 796,363
Remaining unrestricted equity, liquid assets	45,200
RLI Ins.
Statutory information and dividend restrictions
Authorized control level RBC	123,600	118,400	90,600
Selected information for insurance subsidiaries
Consolidated surplus, statutory basis	$ 865,300	849,300	859,200
Dividend restriction as percentage of policyholder surplus	10.00%
Payments of Ordinary Dividends	$ 125,000	185,000	40,000
Extraordinary dividend paid after seeking and receiving approval from the Illinois regulatory authorities	0	0	0
Non restricted net assets	58,200
Mt. Hawley Insurance Company
Statutory information and dividend restrictions
Cost basis of RLI stock held by the subsidiary	64,600
Insurance subsidiaries
Selected information for insurance subsidiaries
Consolidated net income, statutory basis	178,502	176,664	122,550
Consolidated surplus, statutory basis	865,268	849,297	859,221
Common Stock | Mt. Hawley Insurance Company
Selected information for insurance subsidiaries
Consolidated surplus, statutory basis	$ 117,500	$ 86,100	$ 64,000