STOCK BASED COMPENSATION (Details) (USD $)	3 Months Ended					3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan	Mar. 31, 2013 Omnibus Stock Plan	Dec. 31, 2005 Omnibus Stock Plan	Mar. 31, 2013 RLI Corp. Long-Term Incentive Plan (LTIP)	Mar. 31, 2013 Outside Directors' Stock Option Plan (the director's plan)
Stock based compensation
Shares authorized for grant					1,500,000	2,000,000
Stock options granted to date (in shares)				1,228,726		869,400
Award vesting period			5 years
Term of options			8 years
Term of options granted prior to 2009			10 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)	1,472,602	1,280,866
Options granted (in shares)	29,500	34,500				29,500
Options exercised (in shares)	(42,573)	(50,140)
Options canceled/forfeited (in shares)	(19,410)	(2,010)
Outstanding options at the end of the period (in shares)	1,440,119	1,263,216
Exercisable options at the end of the period (in shares)	655,389	568,016
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)	$ 44.45	$ 43.23
Options granted (in dollars per share)	$ 69.48	$ 72.61
Options exercised (in dollars per share)	$ 36.09	$ 41.28
Options canceled/forfeited (in dollars per share)	$ 50.89	$ 45.10
Outstanding options at the end of the period (in dollars per share)	$ 45.12	$ 44.11
Exercisable options at the end of the period (in dollars per share)	$ 36.97	$ 39.80
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding	5 years 3 months 11 days	5 years 6 months 11 days
Weighted-average remaining contractual term of exercisable options	4 years 1 month 6 days	4 years 5 months 8 days
Aggregate Intrinsic Value
Options exercised (in dollars)	$ 1,408,000	$ 1,544,000
Outstanding options at the end of the period (in dollars)	38,492,000	34,810,000
Exercisable options at the end of the period (in dollars)	22,857,000	18,087,000
Average fair value of options granted (in dollars per share)	$ 13.37	$ 14.68
Stock-based compensation expenses (in dollars)	800,000	800,000					100,000
Income tax benefit from stock-based compensation (in dollars)	300,000	300,000
Unrecognized stock-based compensation expense (in dollars)	$ 3,700,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)	$ 13.37	$ 14.68
Risk-free interest rates (as a percent)	1.14%	1.00%
Dividend yield (as a percent)	1.98%	1.92%
Expected volatility (as a percent)	25.52%	25.79%
Expected option life	5 years 7 months 24 days	5 years 9 months 18 days
Period for which annualized dividends is considered to calculate dividend yield	5 years
Share Based Compensation, Arrangement by Share Based Payment Award, Restriction on Sale of Shares Period							1 year
Restricted shares issued							1,160