STOCK BASED COMPENSATION (Details) (USD $)	3 Months Ended					3 Months Ended	47 Months Ended	12 Months Ended	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan	Mar. 31, 2014 Omnibus Stock Plan	Dec. 31, 2005 Omnibus Stock Plan	Mar. 31, 2014 RLI Corp. Long-Term Incentive Plan (LTIP)	Mar. 31, 2014 RLI Corp. Long-Term Incentive Plan (LTIP)	Dec. 31, 2013 RLI Corp. Long-Term Incentive Plan (LTIP) Outside director	Mar. 31, 2014 Outside Directors' Stock Option Plan (the director's plan)
Stock based compensation
Shares authorized for grant					3,000,000	4,000,000	4,000,000
Stock options granted to date (in shares)				2,457,644			2,363,000
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
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)	2,595,084	2,945,204
Options granted (in shares)	50,500	59,000				50,500
Options exercised (in shares)	(4,700)	(85,146)
Options canceled/forfeited (in shares)	(740)	(38,820)
Outstanding options at the end of the period (in shares)	2,640,144	2,880,238
Exercisable options at the end of the period (in shares)	972,284	1,310,778
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)	$ 26.04	$ 22.23
Options granted (in dollars per share)	$ 40.43	$ 34.74
Options exercised (in dollars per share)	$ 18.53	$ 18.05
Options canceled/forfeited (in dollars per share)	$ 16.51	$ 25.45
Outstanding options at the end of the period (in dollars per share)	$ 26.33	$ 22.56
Exercisable options at the end of the period (in dollars per share)	$ 20.66	$ 18.49
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding	5 years 4 months 10 days	5 years 3 months 11 days
Weighted-average remaining contractual term of exercisable options	4 years 25 days	4 years 1 month 6 days
Aggregate Intrinsic Value
Options exercised (in dollars)	$ 115,000	$ 1,408,000
Outstanding options at the end of the period (in dollars)	47,359,000	38,492,000
Exercisable options at the end of the period (in dollars)	22,927,000	22,857,000
Weighted average fair value of options granted (in dollars per share)	$ 8.14	$ 6.69
Stock-based compensation expenses (in dollars)	900,000	800,000							100,000
Income tax benefit from stock-based compensation (in dollars)	300,000	300,000
Unrecognized stock-based compensation expense (in dollars)	4,100,000
Weighted average grant date assumptions and weighted average fair value
Weighted average fair value of options granted (in dollars per share)	$ 8.14	$ 6.69
Risk-free interest rates (as a percent)	1.76%	1.14%
Dividend yield (as a percent)	1.92%	1.98%
Expected volatility (as a percent)	25.20%	25.52%
Expected option life	5 years 7 months 28 days	5 years 7 months 24 days
Period for which annualized dividends is considered to calculate dividend yield	5 years
Restricted shares issued per recipient (in dollars)								$ 10,000
Restriction period									1 year
Restricted shares issued									2,097