SCHEDULE IV-REINSURANCE (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
RLI Insurance Group Premiums earned
Direct amount									$ 981,121	$ 896,234	$ 835,118
Ceded to other companies									182,183	146,794	129,702
Assumed from other companies									40,173	41,926	32,521
Net amount	$ 215,626	$ 211,255	$ 207,541	$ 204,689	$ 204,002	$ 200,815	$ 196,522	$ 190,027	$ 839,111	$ 791,366	$ 737,937
Percentage of amount assumed to net									4.80%	5.30%	4.40%
Casualty segment
RLI Insurance Group Premiums earned
Direct amount									$ 641,159	$ 578,643	$ 536,085
Ceded to other companies									122,452	96,639	86,190
Assumed from other companies									39,751	41,468	28,708
Net amount									$ 558,458	$ 523,472	$ 478,603
Percentage of amount assumed to net									7.10%	7.90%	6.00%
Property segment
RLI Insurance Group Premiums earned
Direct amount									$ 217,657	$ 193,855	$ 172,668
Ceded to other companies									53,810	44,634	37,607
Assumed from other companies									175	40	3,285
Net amount									$ 164,022	$ 149,261	$ 138,346
Percentage of amount assumed to net									0.10%	0.00%	2.40%
Surety segment
RLI Insurance Group Premiums earned
Direct amount									$ 122,305	$ 123,736	$ 126,365
Ceded to other companies									5,921	5,521	5,905
Assumed from other companies									247	418	528
Net amount									$ 116,631	$ 118,633	$ 120,988
Percentage of amount assumed to net									0.20%	0.40%	0.40%