STOCK BASED COMPENSATION (Details) (USD $)	3 Months Ended		60 Months Ended	59 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	May 06, 2010	Mar. 31, 2015
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)	2,892,717	2,595,084
Options granted (in shares)	53,500	50,500
Options exercised (in shares)	(148,952)	(4,700)
Options canceled/forfeited (in shares)	(3,760)	(740)
Outstanding options at the end of the period (in shares)	2,793,505	2,640,144		2,793,505
Exercisable options at the end of the period (in shares)	1,141,065	972,284		1,141,065
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)	$ 26.65	$ 26.04
Options granted (in dollars per share)	$ 48.82	$ 40.43
Options exercised (in dollars per share)	$ 16.66	$ 18.53
Options canceled/forfeited (in dollars per share)	$ 30.09	$ 16.51
Outstanding options at the end of the period (in dollars per share)	$ 27.60	$ 26.33		27.60
Exercisable options at the end of the period (in dollars per share)	$ 20.84	$ 20.66		20.84
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding	5 years 1 month 10 days	5 years 4 months 10 days
Weighted-average remaining contractual term of exercisable options	3 years 10 months 24 days	4 years 26 days
Aggregate Intrinsic Value
Options exercised (in dollars)	$ 4,908,000	$ 115,000
Outstanding options at the end of the period (in dollars)	69,299,000	47,359,000		69,299,000
Exercisable options at the end of the period (in dollars)	36,022,000	22,927,000		36,022,000
Weighted-average fair value of grants (in dollars per share)	$ 8.97	$ 8.14
Stock-based compensation expenses (in dollars)	900,000	900,000
Income tax benefit from stock-based compensation (in dollars)	300,000	300,000
Unrecognized stock-based compensation expense (in dollars)	4,300,000			4,300,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)	$ 8.97	$ 8.14
Risk-free interest rates (as a percent)	1.19%	1.76%
Dividend yield (as a percent)	1.80%	1.92%
Expected volatility (as a percent)	22.98%	25.20%
Expected option life	5 years 5 months 5 days	5 years 7 months 28 days
Period for which annualized dividends is considered to calculate dividend yield	5 years
RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	5 years
Term of options	8 years
Omnibus Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for grant			3,000,000
Weighted Number of Options Outstanding
Options granted (in shares)			2,458,059
RLI Corp. Long-Term Incentive Plan (LTIP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for grant	4,000,000			4,000,000
Age and period of service of the participant to be eligible for retirement	75 years
Weighted Number of Options Outstanding
Options granted (in shares)	53,500			2,878,000
RLI Corp. Long-Term Incentive Plan (LTIP) | Outside director
Weighted average grant date assumptions and weighted average fair value
Restricted shares issued per recipient (in dollars)		10,000
Outside Directors' Stock Option Plan (the director's plan)
Aggregate Intrinsic Value
Stock-based compensation expenses (in dollars)		$ 100,000
Weighted average grant date assumptions and weighted average fair value
Restriction period		1 year
Restricted shares issued		2,097