EMPLOYEE BENEFITS - Stock Options and Stock Plans (Details) - USD ($)							1 Months Ended			3 Months Ended	12 Months Ended			20 Months Ended	60 Months Ended
	Dec. 23, 2016	Nov. 10, 2016	Dec. 22, 2015	Nov. 12, 2015	Dec. 22, 2014	Nov. 13, 2014	May 31, 2016	May 31, 2015	May 31, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	May 06, 2015	May 05, 2010	May 07, 2015
Stock options and stock plans
Dividend paid (in dollars per share)											$ 2.79	$ 2.75	$ 3.71
Nonqualified options as percentage of options outstanding													99.00%
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)										2,595,084	2,582,220	2,892,717	2,595,084
Options granted (in shares)							345,750	412,000	369,500		440,750	563,500	512,000
Special dividend (in shares)													415
Options exercised (in shares)											(756,380)	(865,957)	(214,042)
Options canceled/forfeited (in shares)											(59,480)	(8,040)	(740)
Outstanding options at the end of the period (in shares)											2,207,110	2,582,220	2,892,717	2,207,110
Exercisable options at the end of the period (in shares)											862,605	899,680	1,238,257	862,605
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)										$ 26.04	$ 32.42	$ 26.65	$ 26.04
Options granted (in dollars per share)											63.54	49.01	40.46
Special dividend (in dollars per share)													16.72
Options exercised (in dollars per share)											24.87	19.23	18.93
Options canceled/forfeited (in dollars per share)											44.39	32.12	13.51
Outstanding options at the end of the period (in dollars per share)											40.90	32.42	26.65	$ 40.90
Exercisable options at the end of the period (in dollars per share)											$ 31.23	$ 23.60	$ 20.02	$ 31.23
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding											4 years 11 months 5 days	5 years 3 months 26 days	5 years 2 months 5 days
Weighted-average remaining contractual term of exercisable options											3 years 3 months 7 days	3 years 11 months 23 days	3 years 11 months 9 days
Aggregate Intrinsic Value
Options exercised (in dollars)											$ 31,328,000	$ 32,135,000	$ 6,164,000
Outstanding options at the end of the period (in dollars)											49,531,000	75,725,000	65,809,000	$ 49,531,000
Exercisable options at the end of the period (in dollars)											$ 27,523,000	$ 34,327,000	$ 36,385,000	27,523,000
Weighted-average fair value of options granted (in dollars per share)							$ 11.42	$ 8.94	$ 7.71		$ 11.38	$ 9.25	$ 7.89
Stock-based compensation expenses (in dollars)											$ 4,100,000	$ 4,100,000	$ 3,900,000
Income tax benefit from stock-based compensation (in dollars)											1,400,000	1,400,000	1,400,000
Unrecognized stock-based compensation expense (in dollars)											$ 6,200,000	$ 5,700,000	$ 4,800,000	$ 6,200,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)							$ 11.42	$ 8.94	$ 7.71		$ 11.38	$ 9.25	$ 7.89
Risk-free interest rates (as a percent)											1.21%	1.54%	1.70%
Dividend yield (as a percent)											1.61%	1.81%	1.94%
Expected volatility (as a percent)											23.06%	22.91%	23.17%
Expected option life											5 years 15 days	5 years 2 months 16 days	5 years 2 months 9 days
Period for which annualized dividends is considered to calculate dividend yield											5 years
Special Cash Dividends Member
Stock options and stock plans
Dividend declared (in dollars per share)		$ 2.00		$ 2.00		$ 3.00
Dividend paid (in dollars per share)	$ 2.00		$ 2.00		$ 3.00
RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan
Stock options and stock plans
Award vesting period											5 years
Age and period of service of the participant to be eligible for retirement											75 years
Omnibus Stock Plan
Stock options and stock plans
Shares authorized for grant																3,000,000
Stock options granted to date (in shares)																2,458,059
RLI Corp. Long-Term Incentive Plan (2010 LTIP)
Stock options and stock plans
Shares authorized for grant																	4,000,000
Stock options granted to date (in shares)															2,878,000
RLI Corp. Long-Term Incentive Plan (2015 LITP)
Stock options and stock plans
Shares authorized for grant											4,000,000			4,000,000
Stock options granted to date (in shares)														950,750
Weighted Number of Options Outstanding
Options granted (in shares)											440,750
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
Restricted shares issued per recipient (in dollars)													$ 10,000
Restricted shares issued										2,097
Restriction period										1 year