EMPLOYEE BENEFITS - Stock Options and Stock Plans (Details) - USD ($)	1 Months Ended			12 Months Ended			24 Months Ended	44 Months Ended	60 Months Ended
	May 31, 2018	May 31, 2017	May 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2018	May 06, 2015	May 07, 2015
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)				2,257,015	2,207,110	2,582,220	2,207,110
Options granted (in shares)	330,750	384,750	345,750	432,000	482,375	440,750
Options exercised (in shares)				(705,785)	(390,870)	(756,380)
Options canceled/forfeited (in shares)				(18,350)	(41,600)	(59,480)
Outstanding options at the end of the period (in shares)				1,964,880	2,257,015	2,207,110	1,964,880	1,964,880
Exercisable options at the end of the period (in shares)				724,730	975,055	862,605	724,730	724,730
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)				$ 46.80	$ 40.90	$ 32.42	$ 40.90
Options granted (in dollars per share)				64.91	57.12	63.54
Options exercised (in dollars per share)				36.81	26.07	24.87
Options canceled/forfeited (in dollars per share)				60.84	48.30	44.39
Outstanding options at the end of the period (in dollars per share)				54.24	46.80	40.90	54.24	$ 54.24
Exercisable options at the end of the period (in dollars per share)				$ 46.62	$ 38.66	$ 31.23	$ 46.62	$ 46.62
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding				5 years 3 months	4 years 10 months 17 days	4 years 11 months 5 days
Weighted-average remaining contractual term of exercisable options				3 years 9 months 15 days	3 years 3 months	3 years 3 months 7 days
Aggregate Intrinsic Value
Options exercised (in dollars)				$ 24,304,000	$ 12,779,000	$ 31,328,000
Outstanding options at the end of the period (in dollars)				29,317,000	32,620,000	49,531,000	$ 29,317,000	$ 29,317,000
Exercisable options at the end of the period (in dollars)				$ 16,212,000	$ 21,780,000	$ 27,523,000	16,212,000	16,212,000
Weighted-average fair value of options granted (in dollars per share)	$ 10.31	$ 7.91	$ 11.42	$ 10.58	$ 8.00	$ 11.38
Stock-based compensation expenses (in dollars)				$ 4,500,000	$ 4,400,000	$ 4,100,000
Income tax benefit from stock-based compensation (in dollars)				900,000	1,500,000	1,400,000
Unrecognized stock-based compensation expense (in dollars)				$ 5,600,000	$ 5,700,000	$ 6,200,000	$ 5,600,000	$ 5,600,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)	$ 10.31	$ 7.91	$ 11.42	$ 10.58	$ 8.00	$ 11.38
Risk-free interest rates (as a percent)				2.72%	1.90%	1.21%
Dividend yield (as a percent)				2.98%	3.60%	1.61%
Expected volatility (as a percent)				22.87%	22.95%	23.06%
Expected option life				5 years 26 days	5 years 18 days	5 years 15 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
Stock options and stock plans
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
RLI Corp. Long-Term Incentive Plan (2010 LTIP)
Stock options and stock plans
Shares authorized for grant										4,000,000
Stock options granted to date (in shares)									2,878,000
RLI Corp. Long-Term Incentive Plan (2015 LTIP)
Stock options and stock plans
Shares authorized for grant				4,000,000			4,000,000	4,000,000
Stock options granted to date (in shares)								1,903,630
Weighted Number of Options Outstanding
Options granted (in shares)				455,055
Restricted Stock Units (RSUs) [Member] | Outside director
Stock options and stock plans
Award vesting period				1 year
Aggregate Intrinsic Value
Stock-based compensation expenses (in dollars)				$ 300,000
Unrecognized stock-based compensation expense (in dollars)				$ 200,000			$ 200,000	$ 200,000
Weighted average grant date assumptions and weighted average fair value
RSUs Granted				8,430
RSUs Outstanding				8,629			8,629	8,629
Grant date fair value of RSUs				$ 50,000
Restricted Stock Units (RSUs) [Member] | RLI Corp. Long-Term Incentive Plan (2015 LTIP)
Stock options and stock plans
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)				$ 600,000	$ 400,000
Unrecognized stock-based compensation expense (in dollars)				$ 700,000			$ 700,000	$ 700,000
Weighted average grant date assumptions and weighted average fair value
RSUs Granted				14,625			30,075
RSUs Outstanding				30,567			30,567	30,567