STOCK BASED COMPENSATION (Details) (USD $)	3 Months Ended		6 Months Ended					6 Months Ended	38 Months Ended	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan	Jun. 30, 2013 Omnibus Stock Plan	Dec. 31, 2005 Omnibus Stock Plan	Jun. 30, 2013 RLI Corp. Long-Term Incentive Plan (LTIP)	Jun. 30, 2013 RLI Corp. Long-Term Incentive Plan (LTIP)	Mar. 31, 2013 RLI Corp. Long-Term Incentive Plan (LTIP) Outside director	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	2,000,000
Stock options granted to date (in shares)						1,228,726			1,105,750
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)			265,850	253,400				265,850
Options exercised (in shares)			(153,262)	(61,173)
Options canceled/forfeited (in shares)			(20,760)	(2,710)
Outstanding options at the end of the period (in shares)	1,564,430	1,470,383	1,564,430	1,470,383
Exercisable options at the end of the period (in shares)	737,180	725,363	737,180	725,363
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)			$ 71.58	$ 69.23
Options exercised (in dollars per share)			$ 33.95	$ 40.52
Options canceled/forfeited (in dollars per share)			$ 50.93	$ 47.33
Outstanding options at the end of the period (in dollars per share)	$ 50.00	$ 47.82	$ 50.00	$ 47.82
Exercisable options at the end of the period (in dollars per share)	$ 39.66	$ 40.59	$ 39.66	$ 40.59
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding			5 years 7 months 2 days	5 years 8 months 5 days
Weighted-average remaining contractual term of exercisable options			4 years 4 months 17 days	4 years 6 months 18 days
Aggregate Intrinsic Value
Options exercised (in dollars)			$ 5,942,000	$ 1,885,000
Outstanding options at the end of the period (in dollars)	41,315,000	30,230,000	41,315,000	30,230,000
Exercisable options at the end of the period (in dollars)	27,095,000	20,026,000	27,095,000	20,026,000
Average fair value of options granted (in dollars per share)			$ 12.99	$ 13.29
Stock-based compensation expenses (in dollars)	1,000,000	900,000	1,800,000	1,800,000							100,000
Income tax benefit from stock-based compensation (in dollars)	300,000	300,000	600,000	600,000
Unrecognized stock-based compensation expense (in dollars)	5,600,000		5,600,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)			$ 12.99	$ 13.29
Risk-free interest rates (as a percent)			0.74%	0.95%
Dividend yield (as a percent)			2.00%	1.93%
Expected volatility (as a percent)			25.41%	25.64%
Expected option life			5 years 2 months 23 days	5 years 4 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											1,160