STOCK BASED COMPENSATION (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended	53 Months Ended	60 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Sep. 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)			324,525	401,250
Options exercised (in shares)			(493,615)	(685,810)
Options canceled/forfeited (in shares)			(59,050)	(15,600)
Outstanding options at the end of the period (in shares)	1,736,740	1,956,855	1,736,740	1,956,855	1,964,880	1,736,740
Exercisable options at the end of the period (in shares)	622,290	678,905	622,290	678,905		622,290
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)			81.27	64.31
Options exercised (in dollars per share)			45.61	36.72
Options canceled/forfeited (in dollars per share)			60.92	61.44
Outstanding options at the end of the period (in dollars per share)	$ 61.52	$ 53.81	61.52	53.81	$ 54.24	$ 61.52
Exercisable options at the end of the period (in dollars per share)	$ 52.78	$ 45.60	$ 52.78	$ 45.60		$ 52.78
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding			5 years 5 months 4 days	5 years 6 months
Weighted-average remaining contractual term of exercisable options			4 years 10 days	4 years 10 days
Aggregate Intrinsic Value
Options exercised (in dollars)			$ 18,157,000	$ 23,603,000
Outstanding options at the end of the period (in dollars)	$ 54,515,000	$ 48,476,000	54,515,000	48,476,000		$ 54,515,000
Exercisable options at the end of the period (in dollars)	24,975,000	22,388,000	$ 24,975,000	$ 22,388,000		24,975,000
Weighted-average fair value of grants (in dollars per share)			$ 13.47	$ 10.38
Stock-based compensation expenses (in dollars)	1,200,000	1,200,000	$ 3,600,000	$ 3,400,000
Income tax benefit from stock-based compensation (in dollars)	200,000	200,000	700,000	$ 700,000
Unrecognized stock-based compensation expense (in dollars)	5,800,000		$ 5,800,000			5,800,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)			$ 13.47	$ 10.38
Risk-free interest rates (as a percent)			2.41%	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 12 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)	300,000	200,000	$ 600,000	$ 400,000
Unrecognized stock-based compensation expense (in dollars)	$ 1,200,000		$ 1,200,000			$ 1,200,000
Weighted average grant date assumptions and weighted average fair value
RSUs Granted			15,275			45,350
RSUs Outstanding	43,530		43,530			43,530
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)	$ 100,000	$ 100,000	$ 500,000	$ 200,000
Unrecognized stock-based compensation expense (in dollars)	$ 300,000		$ 300,000			$ 300,000
Weighted average grant date assumptions and weighted average fair value
RSUs Granted			6,655			15,085
RSUs Outstanding	6,081		6,081			6,081
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)			346,455			2,250,085