Schedule IV-Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
RLI Insurance Group Premiums earned
Direct amount	$ 1,895,065	$ 1,671,044	$ 1,425,165
Ceded to other companies	394,829	405,113	316,409
Assumed from other companies	26,170	28,375	35,680
Net	$ 1,526,406	$ 1,294,306	$ 1,144,436
Percentage of amount assumed to net	1.70%	2.20%	3.10%
Casualty Segment
RLI Insurance Group Premiums earned
Direct amount	$ 1,009,937	$ 909,081	$ 863,530
Ceded to other companies	182,226	178,018	186,469
Assumed from other companies	25,126	27,283	34,771
Net	$ 852,837	$ 758,346	$ 711,832
Percentage of amount assumed to net	2.90%	3.60%	4.90%
Property Segment
RLI Insurance Group Premiums earned
Direct amount	$ 729,018	$ 619,250	$ 430,010
Ceded to other companies	198,206	218,265	122,415
Assumed from other companies	572	545	291
Net	$ 531,384	$ 401,530	$ 307,886
Percentage of amount assumed to net	0.10%	0.10%	0.10%
Surety Segment
RLI Insurance Group Premiums earned
Direct amount	$ 156,110	$ 142,713	$ 131,625
Ceded to other companies	14,397	8,830	7,525
Assumed from other companies	472	547	618
Net	$ 142,185	$ 134,430	$ 124,718
Percentage of amount assumed to net	0.30%	0.40%	0.50%