EMPLOYEE BENEFITS - Stock Options and Stock Plans (Details) - USD ($) $ / shares in Units, $ in Thousands							1 Months Ended			12 Months Ended			32 Months Ended	60 Months Ended
	Dec. 27, 2017	Nov. 09, 2017	Dec. 23, 2016	Nov. 10, 2016	Dec. 22, 2015	Nov. 12, 2015	May 31, 2017	May 31, 2016	May 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	May 06, 2015	May 05, 2010	May 07, 2015
Stock options and stock plans
Dividend paid (in dollars per share)										$ 2.58	$ 2.79	$ 2.75
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)										2,207,110	2,582,220	2,892,717
Options granted (in shares)							384,750	345,750	412,000	482,375	440,750	563,500
Options exercised (in shares)										(390,870)	(756,380)	(865,957)
Options canceled/forfeited (in shares)										(41,600)	(59,480)	(8,040)
Outstanding options at the end of the period (in shares)										2,257,015	2,207,110	2,582,220	2,257,015
Exercisable options at the end of the period (in shares)										975,055	862,605	899,680	975,055
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)										$ 40.90	$ 32.42	$ 26.65
Options granted (in dollars per share)										57.12	63.54	49.01
Options exercised (in dollars per share)										26.07	24.87	19.23
Options canceled/forfeited (in dollars per share)										48.30	44.39	32.12
Outstanding options at the end of the period (in dollars per share)										46.80	40.90	32.42	$ 46.80
Exercisable options at the end of the period (in dollars per share)										$ 38.66	$ 31.23	$ 23.60	$ 38.66
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding										4 years 10 months 17 days	4 years 11 months 5 days	5 years 3 months 26 days
Weighted-average remaining contractual term of exercisable options										3 years 3 months	3 years 3 months 7 days	3 years 11 months 23 days
Aggregate Intrinsic Value
Options exercised (in dollars)										$ 12,779	$ 31,328	$ 32,135
Outstanding options at the end of the period (in dollars)										32,620	49,531	75,725	$ 32,620
Exercisable options at the end of the period (in dollars)										$ 21,780	$ 27,523	$ 34,327	21,780
Weighted-average fair value of options granted (in dollars per share)							$ 7.91	$ 11.42	$ 8.94	$ 8.00	$ 11.38	$ 9.25
Stock-based compensation expenses (in dollars)										$ 4,400	$ 4,100	$ 4,100
Income tax benefit from stock-based compensation (in dollars)										1,500	1,400	1,400
Unrecognized stock-based compensation expense (in dollars)										$ 5,700	$ 6,200	$ 5,700	$ 5,700
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)							$ 7.91	$ 11.42	$ 8.94	$ 8.00	$ 11.38	$ 9.25
Risk-free interest rates (as a percent)										1.90%	1.21%	1.54%
Dividend yield (as a percent)										3.60%	1.61%	1.81%
Expected volatility (as a percent)										22.95%	23.06%	22.91%
Expected option life										5 years 18 days	5 years 15 days	5 years 2 months 16 days
Period for which annualized dividends is considered to calculate dividend yield										5 years
Special Cash Dividends Member
Stock options and stock plans
Dividend declared (in dollars per share)		$ 1.75		$ 2.00		$ 2.00
Dividend paid (in dollars per share)	$ 1.75		$ 2.00		$ 2.00
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
Omnibus Stock Plan
Stock options and stock plans
Shares authorized for grant															3,000,000
Stock options granted to date (in shares)															2,458,059
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 LITP)
Stock options and stock plans
Shares authorized for grant										4,000,000			4,000,000
Stock options granted to date (in shares)													1,448,575
Weighted Number of Options Outstanding
Options granted (in shares)										497,825
Restricted Stock Units (RSUs) [Member] | RLI Corp. Long-Term Incentive Plan (2015 LITP)
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)										$ 400
Unrecognized stock-based compensation expense (in dollars)										$ 500			$ 500
Weighted average grant date assumptions and weighted average fair value
RSUs Granted										15,450
RSUs Outstanding										15,300			15,300
Weighted average grant date fair value of RSUs										$ 56.71