STOCK BASED COMPENSATION (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended	50 Months Ended	60 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Jun. 30, 2019	May 06, 2015
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)			1,964,880	2,257,015	2,257,015
Options granted (in shares)			292,150	367,500
Options exercised (in shares)			(414,470)	(337,000)
Options canceled/forfeited (in shares)			(33,600)	(5,200)
Outstanding options at the end of the period (in shares)	1,808,960	2,282,315	1,808,960	2,282,315	1,964,880	1,808,960
Exercisable options at the end of the period (in shares)	672,110	996,265	672,110	996,265		672,110
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)			$ 54.24	$ 46.80	$ 46.80
Options granted (in dollars per share)			80.36	63.27
Options exercised (in dollars per share)			44.70	31.60
Options canceled/forfeited (in dollars per share)			57.61	62.04
Outstanding options at the end of the period (in dollars per share)	$ 60.59	$ 51.66	60.59	51.66	$ 54.24	$ 60.59
Exercisable options at the end of the period (in dollars per share)	$ 52.17	$ 44.04	$ 52.17	$ 44.04		$ 52.17
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding			5 years 6 months 25 days	5 years 4 months 2 days
Weighted-average remaining contractual term of exercisable options			4 years 2 months 15 days	3 years 10 months 20 days
Aggregate Intrinsic Value
Options exercised (in dollars)			$ 14,941,000	$ 11,151,000
Outstanding options at the end of the period (in dollars)	$ 45,448,000	$ 33,224,000	45,448,000	33,224,000		$ 45,448,000
Exercisable options at the end of the period (in dollars)	22,544,000	22,089,000	$ 22,544,000	$ 22,089,000		22,544,000
Weighted-average fair value of grants (in dollars per share)			$ 13.33	$ 10.20
Stock-based compensation expenses (in dollars)	1,400,000	1,300,000	$ 2,400,000	$ 2,300,000
Income tax benefit from stock-based compensation (in dollars)	300,000	300,000	500,000	$ 500,000
Unrecognized stock-based compensation expense (in dollars)	6,700,000		$ 6,700,000			6,700,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)			$ 13.33	$ 10.20
Risk-free interest rates (as a percent)			2.42%	2.69%
Dividend yield (as a percent)			2.69%	3.15%
Expected volatility (as a percent)			22.71%	22.88%
Expected option life			4 years 11 months 15 days	5 years 21 days
Restricted Stock Units (RSUs) [Member]
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)	200,000	100,000	$ 300,000	$ 200,000
Unrecognized stock-based compensation expense (in dollars)	$ 1,500,000		$ 1,500,000			$ 1,500,000
Weighted average grant date assumptions and weighted average fair value
RSUs Granted			15,275			45,350
RSUs Outstanding	44,431		44,431			44,431
Outside director | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period			1 year
Aggregate Intrinsic Value
Stock-based compensation expenses (in dollars)	$ 200,000	$ 100,000	$ 300,000	$ 100,000
Unrecognized stock-based compensation expense (in dollars)	$ 400,000		$ 400,000			$ 400,000
Weighted average grant date assumptions and weighted average fair value
RSUs Granted			6,655			15,085
RSUs Outstanding	7,402		7,402			7,402
Grant date fair value of RSUs			$ 50,000		$ 50,000
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			4,000,000
Weighted Number of Options Outstanding
Options granted (in shares)			314,080			2,217,710