STOCK BASED COMPENSATION (Details) (USD $)	3 Months Ended					3 Months Ended
	Mar. 31, 2012 Y	Mar. 31, 2011 Y	Mar. 31, 2012 RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan	Mar. 31, 2012 Omnibus Stock Plan	Dec. 31, 2005 Omnibus Stock Plan	Mar. 31, 2012 RLI Corp. Long-Term Incentive Plan (LTIP) Y
Stock based compensation
Shares authorized for grant					1,500,000	2,000,000
Stock options granted to date (in shares)				1,228,139		539,500
Award vesting period (in years)			5 years
Term of options (in years)			P8Y
Term of options granted prior to 2009 (in years)			P10Y
Age and period of service of the participant to be eligible for retirement (in years)						75
Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)	1,280,866	1,524,982
Options granted (in shares)	34,500	19,950				34,500
Options exercised (in shares)	(50,140)	(177,857)
Options cancelled/forfeited (in shares)	(2,010)	(7,130)
Outstanding options at the end of the period (in shares)	1,263,216	1,359,945
Exercisable options at the end of the period (in shares)	568,016	668,875
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)	$ 43.23	$ 41.32
Options granted (in dollars per share)	$ 72.61	$ 55.28
Options exercised (in dollars per share)	$ 41.28	$ 28.00
Options canceled/forfeited (in dollars per share)	$ 45.10	$ 44.59
Outstanding options at the end of the period (in dollars per share)	$ 44.11	$ 43.25
Exercisable options at the end of the period (in dollars per share)	$ 39.80	$ 40.79
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding (in years)	5.53	5.89
Weighted-average remaining contractual term of exercisable options (in years)	4.44	5.07
Aggregate Intrinsic Value
Options exercised (in dollars)	$ 1,544,000	$ 4,711,000
Outstanding options at the end of the period (in dollars)	34,810,000	19,583,000
Exercisable options at the end of the period (in dollars)	18,087,000	11,274,000
Average fair value of options granted (in dollars per share)	$ 14.68	$ 12.14
Stock-based compensation expenses (in dollars)	800,000	700,000
Income tax benefit from stock-based compensation	300,000	200,000
Unrecognized stock-based compensation expense (in dollars)	$ 3,300,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)	$ 14.68	$ 12.14
Risk-free interest rates (as a percent)	1.00%	2.41%
Dividend yield (as a percent)	1.92%	1.89%
Expected volatility (as a percent)	25.79%	25.85%
Expected option life (in years)	5.8	6.05
Period for which annualized dividends is considered to calculate dividend yield (in years)	5
Period for which volatility is considered to calculate the expected volatility (in years)	1