STOCK BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended		29 Months Ended	60 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	May 06, 2015	May 05, 2010
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)				2,207,110	2,582,220
Options granted (in shares)				442,625	410,250
Options exercised (in shares)				(174,890)	(638,675)
Options canceled/forfeited (in shares)				(41,600)	(8,940)
Outstanding options at the end of the period (in shares)	2,433,245	2,344,855	2,433,245	2,433,245	2,344,855	2,433,245
Exercisable options at the end of the period (in shares)	1,157,785	939,785	1,157,785	1,157,785	939,785	1,157,785
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)				$ 40.90	$ 32.42
Options granted (in dollars per share)				56.97	64.15
Options exercised (in dollars per share)				25.27	23.67
Options canceled/forfeited (in dollars per share)				48.30	41.90
Outstanding options at the end of the period (in dollars per share)	$ 44.82	$ 40.32	$ 44.82	44.82	40.32	$ 44.82
Exercisable options at the end of the period (in dollars per share)	$ 36.30	$ 31.10	$ 36.30	$ 36.30	$ 31.10	$ 36.30
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding				4 years 10 months 2 days	5 years 1 month 13 days
Weighted-average remaining contractual term of exercisable options				3 years 3 months 4 days	3 years 6 months 29 days
Aggregate Intrinsic Value
Options exercised (in dollars)				$ 5,655	$ 27,704
Outstanding options at the end of the period (in dollars)	$ 33,290	$ 65,748	$ 33,290	33,290	65,748	$ 33,290
Exercisable options at the end of the period (in dollars)	25,078	35,016	25,078	$ 25,078	$ 35,016	25,078
Weighted-average fair value of grants (in dollars per share)				$ 7.95	$ 11.47
Stock-based compensation expenses (in dollars)	1,000	1,000		$ 3,000	$ 3,200
Income tax benefit from stock-based compensation (in dollars)	300	$ 400		1,100	$ 1,100
Unrecognized stock-based compensation expense (in dollars)	6,300		6,300	$ 6,300		6,300
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)				$ 7.95	$ 11.47
Risk-free interest rates (as a percent)				1.89%	1.21%
Dividend yield (as a percent)				3.60%	1.61%
Expected volatility (as a percent)				22.95%	23.06%
Expected option life				5 years 18 days	5 years 15 days
Period for which annualized dividends is considered to calculate dividend yield				5 years
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)	100			$ 300
Unrecognized stock-based compensation expense (in dollars)	$ 500		$ 500	$ 500		$ 500
Weighted average grant date assumptions and weighted average fair value
RSUs Granted			15,450
RSUs Outstanding	15,375		15,375	15,375		15,375
Weighted average grant date fair value of RSUs			$ 56.71
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 LITP)
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized for grant	4,000,000		4,000,000	4,000,000		4,000,000
Weighted Number of Options Outstanding
Options granted (in shares)				458,075		1,408,825