STOCK BASED COMPENSATION (Details) (USD $)	3 Months Ended		6 Months Ended				6 Months Ended	18 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 Omnibus Stock Plan	Jun. 30, 2010 Omnibus Stock Plan	Jun. 30, 2011 RLI Corp. Long-Term Incentive Plan (LTIP)	Jun. 30, 2011 RLI Corp. Long-Term Incentive Plan (LTIP)
Stock based compensation
Shares authorized for grant (in shares)						1,500,000	2,000,000	2,000,000
Stock options granted (in shares)			220,950	183,250			220,950
Stock options granted to date (in shares)					1,227,445		220,950	428,000
Award vesting period (in years)			five years
Term of options (in years)			eight years
Term of options granted prior to 2009 (in years)			ten years
Age and period of service of the participant to be eligible for retirement (in years)			75
Number of Options Outstanding
Options outstanding at the beginning of the period (in shares)			1,524,982	1,583,803
Options granted (in shares)			220,950	183,250			220,950
Options exercised (in shares)			(253,816)	(155,439)
Options canceled / forfeited (in shares)			(21,630)	(23,130)
Options outstanding at the end of the period (in shares)	1,470,486	1,588,484	1,470,486	1,588,484
Options exercisable at the end of the period (in shares)	782,016	910,793	782,016	910,793
Weighted Average Exercise Price
Options outstanding at the beginning of the period (in dollars per share)			$ 41.32	$ 44.73
Options granted (in dollars per share)	$ 58.42	$ 55.93	$ 58.42	$ 55.93
Options exercised (in dollars per share)	$ 31.26	$ 31.50	$ 31.26	$ 31.50
Options canceled/forfeited (in dollars per share)	$ 44.83	$ 50.51	$ 44.83	$ 50.51
Options outstanding at the end of the period (in dollars per share)	$ 45.57	$ 47.24	$ 45.57	$ 47.24
Exercisable options at the end of the period (in dollars per share)	$ 42.02	$ 43.59	$ 42.02	$ 43.59
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding (in years)	5.93	5.93	5.93	5.93
Weighted-average remaining contractual term of exercisable options (in years)			5.04	4.87
Aggregate Intrinsic Value
Options exercised (in dollars)			$ 6,265,000	$ 3,784,000
Options outstanding at the end of the period (in dollars)	24,037,000	8,377,000	24,037,000	8,377,000
Exercisable options at the end of the period (in dollars)	15,565,000	8,128,000	15,565,000	8,128,000
Average fair value of options granted (in dollars per share)			$ 12.85	$ 13.37
Stock-based compensation expenses (in dollars)	1,000,000	900,000	1,700,000	1,600,000
Income tax benefit from stock-based compensation (In dollars)	300,000	300,000	600,000	600,000
Unrecognized stock-based compensation expense (in dollars)	$ 4,500,000		$ 4,500,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)			$ 12.85	$ 13.37
Risk-free interest rates (as a percent)			2.35%	2.76%
Dividend yield (as a percent)			1.89%	1.74%
Expected volatility (as a percent)			25.67%	25.91%
Expected option life (in years)			5.50	5.57
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