STOCK BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		17 Months Ended	60 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	May 06, 2015	May 05, 2010
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)			2,582,220	2,892,717
Options granted (in shares)			410,250	517,000
Options exercised (in shares)			(638,675)	(511,187)
Options canceled/forfeited (in shares)			(8,940)	(4,560)
Outstanding options at the end of the period (in shares)	2,344,855	2,893,970	2,344,855	2,893,970	2,344,855
Exercisable options at the end of the period (in shares)	939,785	1,197,470	939,785	1,197,470	939,785
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)			$ 32.42	$ 26.65
Options granted (in dollars per share)			64.15	50.09
Options exercised (in dollars per share)			23.67	18.27
Options canceled/forfeited (in dollars per share)			41.90	31.87
Outstanding options at the end of the period (in dollars per share)	$ 40.32	$ 32.31	40.32	32.31	$ 40.32
Exercisable options at the end of the period (in dollars per share)	$ 31.10	$ 23.89	$ 31.10	$ 23.89	$ 31.10
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding			5 years 1 month 13 days	5 years 3 months 15 days
Weighted-average remaining contractual term of exercisable options			3 years 6 months 29 days	4 years 4 days
Aggregate Intrinsic Value
Options exercised (in dollars)			$ 27,704	$ 17,845
Outstanding options at the end of the period (in dollars)	$ 65,748	$ 61,522	65,748	61,522	$ 65,748
Exercisable options at the end of the period (in dollars)	35,016	35,497	$ 35,016	$ 35,497	35,016
Weighted-average fair value of grants (in dollars per share)			$ 11.47	$ 9.04
Stock-based compensation expenses (in dollars)	1,000	1,100	$ 3,200	$ 3,000
Income tax benefit from stock-based compensation (in dollars)	400	$ 400	1,100	$ 1,100
Unrecognized stock-based compensation expense (in dollars)	$ 7,100		$ 7,100		$ 7,100
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)			$ 11.47	$ 9.04
Risk-free interest rates (as a percent)			1.21%	1.53%
Dividend yield (as a percent)			1.61%	1.81%
Expected volatility (as a percent)			23.06%	22.89%
Expected option life			5 years 15 days	5 years 2 months 16 days
Period for which annualized dividends is considered to calculate dividend yield			5 years
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
Weighted Number of Options Outstanding
Options granted (in shares)			410,250		920,250