STOCK BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended	59 Months Ended	60 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Mar. 31, 2020	May 06, 2015
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	40,750
Options exercised (in shares)	(56,725)	(50,580)
Options canceled/forfeited (in shares)	(300)	(19,100)
Outstanding options at the end of the period (in shares)	1,640,640	1,935,950	1,667,290	1,640,640
Exercisable options at the end of the period (in shares)	614,040	702,250		614,040
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	66.16
Options exercised (in dollars per share)	49.23	33.71
Options canceled/forfeited (in dollars per share)	58.60	57.28
Outstanding options at the end of the period (in dollars per share)	63.58	55.00	$ 62.52	$ 63.58
Exercisable options at the end of the period (in dollars per share)	$ 54.68	$ 47.84		$ 54.68
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding	5 years 1 month 6 days	5 years 1 month 20 days
Weighted-average remaining contractual term of exercisable options	3 years 9 months	3 years 8 months 23 days
Aggregate Intrinsic Value
Options exercised (in dollars)	$ 2,516	$ 1,795
Outstanding options at the end of the period (in dollars)	40,778	32,589		$ 40,778
Exercisable options at the end of the period (in dollars)	$ 20,438	$ 16,794		20,438
Weighted-average fair value of grants (in dollars per share)	$ 14.48	$ 11.35
Stock-based compensation expenses (in dollars)	$ 1,000	$ 1,000
Income tax benefit from stock-based compensation (in dollars)	200	$ 200
Unrecognized stock-based compensation expense (in dollars)	$ 4,200			4,200
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)	$ 14.48	$ 11.35
Risk-free interest rates (as a percent)	1.54%	2.94%
Dividend yield (as a percent)	2.69%	2.98%
Expected volatility (as a percent)	22.68%	22.84%
Expected option life	4 years 11 months 26 days	5 years 1 month 9 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)	$ 200	$ 100
Unrecognized stock-based compensation expense (in dollars)	$ 800			$ 800
Weighted average grant date assumptions and weighted average fair value
RSUs Granted				45,350
RSUs Outstanding	43,708			43,708
Outside director | Maximum
Aggregate Intrinsic Value
Unrecognized stock-based compensation expense (in dollars)	$ 100			$ 100
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)	$ 100	$ 100
Weighted average grant date assumptions and weighted average fair value
RSUs Granted				15,085
RSUs Outstanding	6,182			6,182
Grant date fair value of RSUs	$ 50		$ 50
RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan
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
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
Weighted Number of Options Outstanding
Options granted (in shares)	30,375			2,312,835