EMPLOYEE BENEFITS (Details 2) (USD $)	0 Months Ended					1 Months Ended			12 Months Ended						12 Months Ended	44 Months Ended	12 Months Ended			3 Months Ended
	Nov. 14, 2013	Dec. 20, 2012	Nov. 14, 2012	Dec. 20, 2011	Nov. 17, 2011	May 31, 2013	May 31, 2012	May 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan	Dec. 31, 2013 Omnibus Stock Plan	Dec. 31, 2005 Omnibus Stock Plan	Dec. 31, 2013 RLI Corp. Long-Term Incentive Plan (LTIP)	Dec. 31, 2013 RLI Corp. Long-Term Incentive Plan (LTIP)	Dec. 31, 2013 RLI Corp. Long-Term Incentive Plan (LTIP) Outside director	Dec. 31, 2011 Non-Employee Director's Stock Plan (stock plan)	Dec. 31, 2012 Non-Employee Director's Stock Plan (stock plan) Outside director	Mar. 31, 2013 Outside Directors' Stock Option Plan (the director's plan)
Stock options and stock plans
Shares authorized for grant														3,000,000	4,000,000	4,000,000
Stock options granted to date (in shares)													2,457,644			2,312,500		3,082	0
Award vesting period												5 years
Age and period of service of the participant to be eligible for retirement															75 years
Special dividend declared (in dollars per share)	$ 1.50		$ 2.50		$ 2.50
Special dividend paid (in dollars per share)		$ 2.50		$ 2.50
Nonqualified options as percentage of options outstanding									99.00%
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)									2,945,204	2,561,732	3,049,964
Options granted (in shares)						472,700	437,800	402,000	632,700	669,800	595,900				632,700
Special dividend (in shares)									272	2,172	3,082
Options exercised (in shares)									(935,692)	(282,260)	(1,030,634)
Options canceled/forfeited (in shares)									(47,400)	(6,240)	(56,580)
Outstanding options at the end of the period (in shares)									2,595,084	2,945,204	2,561,732
Exercisable options at the end of the period (in shares)									934,544	1,362,184	1,207,872
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)									$ 22.22	$ 21.62	$ 20.66
Options granted (in dollars per share)									$ 35.67	$ 31.71	$ 27.51							$ 27.48
Special dividend (in dollars per share)									$ 16.38	$ 15.39	$ 16.08
Options exercised (in dollars per share)									$ 17.40	$ 19.03	$ 17.31
Options canceled/forfeited (in dollars per share)									$ 24.86	$ 20.42	$ 20.82
Outstanding options at the end of the period (in dollars per share)									$ 26.04	$ 22.22	$ 21.62
Exercisable options at the end of the period (in dollars per share)									$ 20.36	$ 18.29	$ 19.90
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding									5 years 6 months 22 days	5 years 5 months 5 days	5 years 7 months 13 days
Weighted-average remaining contractual term of exercisable options									4 years 3 months 11 days	4 years 3 months 7 days	4 years 6 months 14 days
Aggregate Intrinsic Value
Options exercised (in dollars)									$ 22,422,000	$ 4,712,000	$ 12,764,000
Outstanding options at the end of the period (in dollars)									58,790,000	29,865,000	37,949,000
Exercisable options at the end of the period (in dollars)									26,474,000	19,135,000	19,965,000
Average fair value of options granted (in dollars per share)						$ 6.47	$ 6.54	$ 6.46	$ 6.88	$ 6.55	$ 6.45
Stock-based compensation expenses (in dollars)									3,800,000	3,600,000	3,500,000							100,000		100,000
Income tax benefit from stock-based compensation (in dollars)									1,300,000	1,300,000	1,200,000
Unrecognized stock-based compensation expense (in dollars)									4,500,000	4,300,000	3,600,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)						$ 6.47	$ 6.54	$ 6.46	$ 6.88	$ 6.55	$ 6.45
Risk-free interest rates (as a percent)									0.87%	0.90%	2.06%
Dividend yield (as a percent)									2.00%	1.93%	1.89%
Expected volatility (as a percent)									25.40%	25.62%	25.68%
Expected option life									5 years 3 months 4 days	5 years 5 months 5 days	5 years 6 months 14 days
Period for which annualized dividends is considered to calculate dividend yield									5 years
Restricted shares issued per recipient (in dollars)																	$ 10,000
Restricted shares issued																				2,320
Restriction period																				1 year