STOCK BASED COMPENSATION (Details) - USD ($)	3 Months Ended		9 Months Ended		41 Months Ended	60 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	May 06, 2015	May 05, 2010
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)			2,257,015	2,207,110
Options granted (in shares)			401,250	442,625
Options exercised (in shares)			(685,810)	(174,890)
Options canceled/forfeited (in shares)			(15,600)	(41,600)
Outstanding options at the end of the period (in shares)	1,956,855	2,433,245	1,956,855	2,433,245	1,956,855
Exercisable options at the end of the period (in shares)	678,905	1,157,785	678,905	1,157,785	678,905
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)			$ 46.80	$ 40.90
Options granted (in dollars per share)			64.31	56.97
Options exercised (in dollars per share)			36.72	25.27
Options canceled/forfeited (in dollars per share)			61.44	48.30
Outstanding options at the end of the period (in dollars per share)	$ 53.81	$ 44.82	53.81	44.82	$ 53.81
Exercisable options at the end of the period (in dollars per share)	$ 45.60	$ 36.30	$ 45.60	$ 36.30	$ 45.60
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding			5 years 6 months	4 years 10 months 2 days
Weighted-average remaining contractual term of exercisable options			4 years 11 days	3 years 3 months 4 days
Aggregate Intrinsic Value
Options exercised (in dollars)			$ 23,603,000	$ 5,655,000
Outstanding options at the end of the period (in dollars)	$ 48,476,000	$ 33,290,000	48,476,000	33,290,000	$ 48,476,000
Exercisable options at the end of the period (in dollars)	22,388,000	25,078,000	$ 22,388,000	$ 25,078,000	22,388,000
Weighted-average fair value of grants (in dollars per share)			$ 10.38	$ 7.95
Stock-based compensation expenses (in dollars)	1,200,000	1,000,000	$ 3,400,000	$ 3,000,000
Income tax benefit from stock-based compensation (in dollars)	200,000	300,000	700,000	$ 1,100,000
Unrecognized stock-based compensation expense (in dollars)	6,200,000		$ 6,200,000		6,200,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)			$ 10.38	$ 7.95
Risk-free interest rates (as a percent)			2.69%	1.89%
Dividend yield (as a percent)			3.15%	3.60%
Expected volatility (as a percent)			22.88%	22.95%
Expected option life			5 years 22 days	5 years 18 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	$ 400,000	$ 300,000
Unrecognized stock-based compensation expense (in dollars)	$ 900,000		$ 900,000		$ 900,000
Weighted average grant date assumptions and weighted average fair value
RSUs Granted			14,625		30,075
RSUs Outstanding	29,325		29,325		29,325
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		$ 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			8,430
Grant date fair value of RSUs			$ 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
Omnibus Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized for grant							3,000,000
Weighted Number of Options Outstanding
Options granted (in shares)							2,458,059
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)			424,305		1,872,880