STOCK BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		23 Months Ended	60 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 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)	23,125	36,500
Options exercised (in shares)	(32,960)	(168,480)
Options canceled/forfeited (in shares)		(5,440)
Outstanding options at the end of the period (in shares)	2,197,275	2,444,800	2,197,275
Exercisable options at the end of the period (in shares)	891,235	966,100	891,235
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)	59.77	61.27
Options exercised (in dollars per share)	33.05	19.00
Options canceled/forfeited (in dollars per share)	0.00	33.69
Outstanding options at the end of the period (in dollars per share)	41.22	33.78	$ 41.22
Exercisable options at the end of the period (in dollars per share)	$ 31.89	$ 27.24	$ 31.89
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding	4 years 8 months 23 days	4 years 11 months 27 days
Weighted-average remaining contractual term of exercisable options	3 years 1 month 13 days	3 years 7 months 13 days
Aggregate Intrinsic Value
Options exercised (in dollars)	$ 891	$ 7,619
Outstanding options at the end of the period (in dollars)	42,954	80,880	$ 42,954
Exercisable options at the end of the period (in dollars)	$ 25,118	$ 38,280	25,118
Weighted-average fair value of grants (in dollars per share)	$ 8.49	$ 11.32
Stock-based compensation expenses (in dollars)	$ 900	$ 900
Income tax benefit from stock-based compensation (in dollars)	300	$ 300
Unrecognized stock-based compensation expense (in dollars)	$ 5,500		$ 5,500
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)	$ 8.49	$ 11.32
Risk-free interest rates (as a percent)	1.95%	1.38%
Dividend yield (as a percent)	3.59%	1.60%
Expected volatility (as a percent)	22.99%	23.11%
Expected option life	5 years 1 month 21 days	5 years 3 months 22 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
Weighted Number of Options Outstanding
Options granted (in shares)	23,125		973,875