STOCK BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		11 Months Ended	60 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 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)	36,500	53,500
Options exercised (in shares)	(168,480)	(148,952)
Options canceled/forfeited (in shares)	(5,440)	(3,760)
Outstanding options at the end of the period (in shares)	2,444,800	2,793,505	2,444,800
Exercisable options at the end of the period (in shares)	966,100	1,141,065	966,100
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)	61.27	48.82
Options exercised (in dollars per share)	19.00	16.66
Options canceled/forfeited (in dollars per share)	33.69	30.09
Outstanding options at the end of the period (in dollars per share)	33.78	27.60	$ 33.78
Exercisable options at the end of the period (in dollars per share)	$ 27.24	$ 20.84	$ 27.24
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding	4 years 11 months 27 days	5 years 1 month 10 days
Weighted-average remaining contractual term of exercisable options	3 years 7 months 13 days	3 years 10 months 24 days
Aggregate Intrinsic Value
Options exercised (in dollars)	$ 7,619	$ 4,908
Outstanding options at the end of the period (in dollars)	80,880	69,299	$ 80,880
Exercisable options at the end of the period (in dollars)	$ 38,280	$ 36,022	38,280
Weighted-average fair value of grants (in dollars per share)	$ 11.32	$ 8.97
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)	$ 5,200		$ 5,200
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)	$ 11.32	$ 8.97
Risk-free interest rates (as a percent)	1.38%	1.19%
Dividend yield (as a percent)	1.60%	1.80%
Expected volatility (as a percent)	23.11%	22.98%
Expected option life	5 years 3 months 22 days	5 years 5 months 5 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)	36,500		546,500