STOCK BASED COMPENSATION (Details) (USD $)	3 Months Ended			9 Months Ended					9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2009	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan	Sep. 30, 2012 Omnibus Stock Plan	Dec. 31, 2005 Omnibus Stock Plan	Sep. 30, 2012 RLI Corp. Long-Term Incentive Plan (LTIP)
Stock based compensation
Shares authorized for grant								1,500,000	2,000,000
Stock options granted to date (in shares)							1,228,139		797,900
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
Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)				1,280,866	1,524,982
Options granted (in shares)				292,900	254,450				292,900
Options exercised (in shares)				(63,721)	(422,457)
Options canceled/forfeited (in shares)				(3,120)	(24,350)
Outstanding options at the end of the period (in shares)	1,506,925	1,332,625		1,506,925	1,332,625
Exercisable options at the end of the period (in shares)	738,945	628,725		738,945	628,725
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)				$ 68.46	$ 58.97
Options exercised (in dollars per share)				$ 40.29	$ 33.34
Options canceled/forfeited (in dollars per share)				$ 45.84	$ 45.84
Outstanding options at the end of the period (in dollars per share)	$ 48.26	$ 47.14		$ 48.26	$ 47.14
Exercisable options at the end of the period (in dollars per share)	$ 40.78	$ 43.62		$ 40.78	$ 43.62
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding				5 years 3 months 25 days	5 years 11 months 16 days
Weighted-average remaining contractual term of exercisable options				4 years 3 months 29 days	5 years 25 days
Aggregate Intrinsic Value
Options exercised (in dollars)				$ 1,966,000	$ 11,220,000
Outstanding options at the end of the period (in dollars)	28,385,000	21,910,000		28,385,000	21,910,000
Exercisable options at the end of the period (in dollars)	19,125,000	12,551,000		19,125,000	12,551,000
Average fair value of options granted (in dollars per share)				$ 13.13	$ 12.97
Stock-based compensation expenses (in dollars)	900,000	1,000,000		2,700,000	2,600,000
Income tax benefit from stock-based compensation	300,000	300,000		900,000	900,000
Unrecognized stock-based compensation expense (in dollars)	$ 4,700,000			$ 4,700,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)				$ 13.13	$ 12.97
Risk-free interest rates (as a percent)				0.90%	2.22%
Dividend yield (as a percent)				1.93%	1.89%
Expected volatility (as a percent)				25.64%	25.67%
Expected option life				5 years 4 months 20 days	5 years 6 months
Period for which annualized dividends is considered to calculate dividend yield				5 years
Period for which volatility is considered to calculate the expected volatility			1 year