Schedule IV-Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
RLI Insurance Group Premiums earned
Direct amount	$ 1,425,165	$ 1,222,346	$ 1,062,608
Ceded to other companies	316,409	272,393	224,512
Assumed from other companies	35,680	30,950	27,651
Net	$ 1,144,436	$ 980,903	$ 865,747
Percentage of amount assumed to net	3.10%	3.20%	3.20%
Casualty Segment
RLI Insurance Group Premiums earned
Direct amount	$ 863,530	$ 788,741	$ 690,718
Ceded to other companies	186,469	185,433	148,271
Assumed from other companies	34,771	30,331	27,074
Net	$ 711,832	$ 633,639	$ 569,521
Percentage of amount assumed to net	4.90%	4.80%	4.80%
Property Segment
RLI Insurance Group Premiums earned
Direct amount	$ 430,010	$ 310,630	$ 253,781
Ceded to other companies	122,415	79,094	70,398
Assumed from other companies	291	301	337
Net	$ 307,886	$ 231,837	$ 183,720
Percentage of amount assumed to net	0.10%	0.10%	0.20%
Surety Segment
RLI Insurance Group Premiums earned
Direct amount	$ 131,625	$ 122,975	$ 118,109
Ceded to other companies	7,525	7,866	5,843
Assumed from other companies	618	318	240
Net	$ 124,718	$ 115,427	$ 112,506
Percentage of amount assumed to net	0.50%	0.30%	0.20%