SCHEDULE IV-REINSURANCE (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
RLI Insurance Group Premiums earned
Direct amount									$ 896,234	$ 835,118	$ 835,294
Ceded to other companies									146,794	129,702	134,572
Assumed from other companies									41,926	32,521	27,886
Net amount	$ 204,002	$ 200,815	$ 196,522	$ 190,027	$ 188,296	$ 182,025	$ 184,331	$ 183,285	$ 791,366	$ 737,937	$ 728,608
Percentage of amount assumed to net									5.30%	4.40%	3.80%
Casualty segment
RLI Insurance Group Premiums earned
Direct amount									$ 578,643	$ 536,085	$ 528,691
Ceded to other companies									96,639	86,190	89,635
Assumed from other companies									41,468	28,708	15,787
Net amount									$ 523,472	$ 478,603	$ 454,843
Percentage of amount assumed to net									7.90%	6.00%	3.50%
Property segment
RLI Insurance Group Premiums earned
Direct amount									$ 193,855	$ 172,668	$ 179,460
Ceded to other companies									44,634	37,607	38,353
Assumed from other companies									40	3,285	11,060
Net amount									$ 149,261	$ 138,346	$ 152,167
Percentage of amount assumed to net									0.00%	2.40%	7.30%
Surety segment
RLI Insurance Group Premiums earned
Direct amount									$ 123,736	$ 126,365	$ 127,143
Ceded to other companies									5,521	5,905	6,584
Assumed from other companies									418	528	1,039
Net amount									$ 118,633	$ 120,988	$ 121,598
Percentage of amount assumed to net									0.40%	0.40%	0.90%