Schedule IV-Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
RLI Insurance Group Premiums earned
Direct amount	$ 1,222,346	$ 1,062,608	$ 981,121
Ceded to other companies	272,393	224,512	182,183
Assumed from other companies	30,950	27,651	40,173
Net	$ 980,903	$ 865,747	$ 839,111
Percentage of amount assumed to net	3.20%	3.20%	4.80%
Casualty Segment
RLI Insurance Group Premiums earned
Direct amount	$ 788,741	$ 690,718	$ 641,159
Ceded to other companies	185,433	148,271	122,452
Assumed from other companies	30,331	27,074	39,751
Net	$ 633,639	$ 569,521	$ 558,458
Percentage of amount assumed to net	4.80%	4.80%	7.10%
Property segment
RLI Insurance Group Premiums earned
Direct amount	$ 310,630	$ 253,781	$ 217,657
Ceded to other companies	79,094	70,398	53,810
Assumed from other companies	301	337	175
Net	$ 231,837	$ 183,720	$ 164,022
Percentage of amount assumed to net	0.10%	0.20%	0.10%
Surety segment
RLI Insurance Group Premiums earned
Direct amount	$ 122,975	$ 118,109	$ 122,305
Ceded to other companies	7,866	5,843	5,921
Assumed from other companies	318	240	247
Net	$ 115,427	$ 112,506	$ 116,631
Percentage of amount assumed to net	0.30%	0.20%	0.20%