STOCK BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended	60 Months Ended	62 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	May 06, 2015	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period			5 years
Term of options			8 years
Age and period of service of the participant to be eligible for retirement			75 years
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)			1,667,290	1,964,880	1,964,880
Options granted (in shares)			30,375	292,150
Options exercised (in shares)			(72,180)	(414,470)
Options canceled/forfeited (in shares)			(2,370)	(33,600)
Outstanding options at the end of the period (in shares)	1,623,115	1,808,960	1,623,115	1,808,960	1,667,290		1,623,115
Exercisable options at the end of the period (in shares)	878,335	672,110	878,335	672,110			878,335
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)			$ 62.52	$ 54.24	$ 54.24
Options granted (in dollars per share)			94.62	80.36
Options exercised (in dollars per share)			45.92	44.70
Options canceled/forfeited (in dollars per share)			66.57	57.61
Outstanding options at the end of the period (in dollars per share)	$ 63.86	$ 60.59	63.86	60.59	$ 62.52		$ 63.86
Exercisable options at the end of the period (in dollars per share)	$ 57.70	$ 52.17	$ 57.70	$ 52.17			$ 57.70
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding			4 years 9 months 25 days	5 years 6 months 25 days
Weighted-average remaining contractual term of exercisable options			3 years 10 months 20 days	4 years 2 months 15 days
Aggregate Intrinsic Value
Options exercised (in dollars)			$ 3,181	$ 14,941
Outstanding options at the end of the period (in dollars)	$ 30,819	$ 45,448	30,819	45,448			$ 30,819
Exercisable options at the end of the period (in dollars)	21,544	22,544	$ 21,544	$ 22,544			21,544
Weighted-average fair value of grants (in dollars per share)			$ 14.48	$ 13.33
Stock-based compensation expenses (in dollars)	600	1,400	$ 1,600	$ 2,400
Income tax benefit from stock-based compensation (in dollars)	100	300	300	$ 500
Unrecognized stock-based compensation expense (in dollars)	3,600		$ 3,600				3,600
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)			$ 14.48	$ 13.33
Risk-free interest rates (as a percent)			1.54%	2.42%
Dividend yield (as a percent)			2.69%	2.69%
Expected volatility (as a percent)			22.68%	22.71%
Expected option life			4 years 11 months 26 days	4 years 11 months 15 days
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period			3 years
Age and period of service of the participant to be eligible for retirement			75 years
Aggregate Intrinsic Value
Stock-based compensation expenses (in dollars)	100	200	$ 300	$ 300
Unrecognized stock-based compensation expense (in dollars)	$ 600		$ 600				$ 600
Weighted average grant date assumptions and weighted average fair value
RSUs Granted							45,350
RSUs Outstanding	28,946		28,946				28,946
Outside director | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period			1 year
Aggregate Intrinsic Value
Stock-based compensation expenses (in dollars)		$ 200	$ 200	$ 300
Weighted average grant date assumptions and weighted average fair value
RSUs Granted							15,085
RSUs Outstanding	0		0				0
Grant date fair value of RSUs					$ 50
Outside director | Restricted Stock Units (RSUs) | Maximum
Aggregate Intrinsic Value
Stock-based compensation expenses (in dollars)	$ 100
RLI Corp. Long-Term Incentive Plan (2010 LTIP)
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized for grant						4,000,000
Weighted Number of Options Outstanding
Options granted (in shares)						2,878,000
RLI Corp. Long-Term Incentive Plan (2015 LTIP)
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized for grant	4,000,000		4,000,000				4,000,000
Weighted Number of Options Outstanding
Options granted (in shares)							2,312,835