The fair value at grant date for nonvested equity-based awards issued during the period on restricted stock units.

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Represents the participant's age and period of service to be considered retirement eligible under the plan.

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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Net number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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STOCK BASED COMPENSATION (Details) - USD ($)	3 Months Ended		12 Months Ended	47 Months Ended	60 Months Ended
STOCK BASED COMPENSATION (Details) - USD ($)	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Mar. 31, 2019	May 06, 2015
Weighted Number of Options Outstanding
Outstanding options at the beginning of the period (in shares)	1,964,880	2,257,015	2,257,015
Options granted (in shares)	40,750	36,750
Options exercised (in shares)	(50,580)	(147,370)
Options canceled/forfeited (in shares)	(19,100)	(4,800)
Outstanding options at the end of the period (in shares)	1,935,950	2,141,595	1,964,880	1,935,950
Exercisable options at the end of the period (in shares)	702,250	853,835		702,250
Weighted Average Exercise Price
Outstanding options at the beginning of the period (in dollars per share)	$ 54.24	$ 46.80	$ 46.80
Options granted (in dollars per share)	66.16	64.42
Options exercised (in dollars per share)	33.71	30.01
Options canceled/forfeited (in dollars per share)	57.28	62.55
Outstanding options at the end of the period (in dollars per share)	55.00	48.22	$ 54.24	$ 55.00
Exercisable options at the end of the period (in dollars per share)	$ 47.84	$ 40.27		$ 47.84
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual term of options outstanding	5 years 1 month 21 days	4 years 10 months 24 days
Weighted-average remaining contractual term of exercisable options	3 years 8 months 23 days	3 years 3 months 26 days
Aggregate Intrinsic Value
Options exercised (in dollars)	$ 1,795,000	$ 5,001,000
Outstanding options at the end of the period (in dollars)	32,589,000	32,883,000		$ 32,589,000
Exercisable options at the end of the period (in dollars)	$ 16,794,000	$ 19,834,000		16,794,000
Weighted-average fair value of grants (in dollars per share)	$ 11.35	$ 9.16
Stock-based compensation expenses (in dollars)	$ 1,000,000	$ 1,000,000
Income tax benefit from stock-based compensation (in dollars)	200,000	$ 200,000
Unrecognized stock-based compensation expense (in dollars)	$ 4,900,000			4,900,000
Weighted average grant date assumptions and weighted average fair value
Weighted-average fair value of grants (in dollars per share)	$ 11.35	$ 9.16
Risk-free interest rates (as a percent)	2.94%	2.02%
Dividend yield (as a percent)	2.98%	3.60%
Expected volatility (as a percent)	22.84%	22.93%
Expected option life	5 years 1 month 10 days	5 years 1 month 17 days
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period	3 years
Age and period of service of the participant to be eligible for retirement	75 years
Aggregate Intrinsic Value
Stock-based compensation expenses (in dollars)	$ 100,000	$ 100,000
Unrecognized stock-based compensation expense (in dollars)	$ 600,000			$ 600,000
Weighted average grant date assumptions and weighted average fair value
RSUs Granted				30,075
RSUs Outstanding	29,793			29,793
Outside director \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period	1 year
Aggregate Intrinsic Value
Stock-based compensation expenses (in dollars)	$ 100,000
Unrecognized stock-based compensation expense (in dollars)	$ 100,000			$ 100,000
Weighted average grant date assumptions and weighted average fair value
RSUs Granted				8,430
RSUs Outstanding	8,656			8,656
Grant date fair value of RSUs			$ 50,000
RLI Corp. Long-Term Incentive Plan (LTIP) and Omnibus Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period	5 years
Term of options	8 years
Age and period of service of the participant to be eligible for retirement	75 years
RLI Corp. Long-Term Incentive Plan (2010 LTIP)
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized for grant					4,000,000
Weighted Number of Options Outstanding
Options granted (in shares)					2,878,000
RLI Corp. Long-Term Incentive Plan (2015 LTIP)
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized for grant	4,000,000			4,000,000
Weighted Number of Options Outstanding
Options granted (in shares)	40,750			1,944,380