Schedule IV-Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
RLI Insurance Group Premiums earned
Direct amount	$ 1,671,044	$ 1,425,165	$ 1,222,346
Ceded to other companies	405,113	316,409	272,393
Assumed from other companies	28,375	35,680	30,950
Net	$ 1,294,306	$ 1,144,436	$ 980,903
Percentage of amount assumed to net	2.20%	3.10%	3.20%
Casualty Segment
RLI Insurance Group Premiums earned
Direct amount	$ 909,081	$ 863,530	$ 788,741
Ceded to other companies	178,018	186,469	185,433
Assumed from other companies	27,283	34,771	30,331
Net	$ 758,346	$ 711,832	$ 633,639
Percentage of amount assumed to net	3.60%	4.90%	4.80%
Property Segment
RLI Insurance Group Premiums earned
Direct amount	$ 619,250	$ 430,010	$ 310,630
Ceded to other companies	218,265	122,415	79,094
Assumed from other companies	545	291	301
Net	$ 401,530	$ 307,886	$ 231,837
Percentage of amount assumed to net	0.10%	0.10%	0.10%
Surety Segment
RLI Insurance Group Premiums earned
Direct amount	$ 142,713	$ 131,625	$ 122,975
Ceded to other companies	8,830	7,525	7,866
Assumed from other companies	547	618	318
Net	$ 134,430	$ 124,718	$ 115,427
Percentage of amount assumed to net	0.40%	0.50%	0.30%