SCHEDULE IV-REINSURANCE (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
RLI Insurance Group Premiums earned
Direct amount	$ 835,294	$ 797,180	$ 781,640
Ceded to other companies	134,572	132,743	167,143
Assumed from other companies	27,886	35,724	72,878
Net amount	$ 728,608	$ 700,161	$ 687,375
Percentage of amount assumed to net	3.80%	5.10%	10.60%
Casualty segment
RLI Insurance Group Premiums earned
Direct amount	$ 528,691	$ 484,435	$ 466,821
Ceded to other companies	89,635	84,311	92,532
Assumed from other companies	15,787	12,124	7,816
Net amount	$ 454,843	$ 412,248	$ 382,105
Percentage of amount assumed to net	3.50%	2.90%	2.00%
Property segment
RLI Insurance Group Premiums earned
Direct amount	$ 179,460	$ 190,678	$ 200,709
Ceded to other companies	38,353	42,731	67,705
Assumed from other companies	11,060	22,977	64,772
Net amount	$ 152,167	$ 170,924	$ 197,776
Percentage of amount assumed to net	7.30%	13.40%	32.80%
Surety segment
RLI Insurance Group Premiums earned
Direct amount	$ 127,143	$ 122,067	$ 114,110
Ceded to other companies	6,584	5,701	6,906
Assumed from other companies	1,039	623	290
Net amount	$ 121,598	$ 116,989	$ 107,494
Percentage of amount assumed to net	0.90%	0.50%	0.30%