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

No definition available.

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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- Definition

Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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- Definition

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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No definition available.

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Number of shares authorized for issuance under share-based payment arrangement.

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

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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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Stock Based Compensation - Stock Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		60 Months Ended	74 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	May 06, 2015	Jun. 30, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted (in shares)			213,271
Aggregate Intrinsic Value, Options exercised			$ 7.3	$ 3.2
RLI Corp. Long-Term Incentive Plan (2010 LTIP)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized for grant					4,000,000
Options granted (in shares)					2,878,000
RLI Corp. Long-Term Incentive Plan (2015 LTIP)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized for grant	4,000,000		4,000,000			4,000,000
Options granted (in shares)			230,051			2,853,420
Stock-based compensation expenses	$ 1.7	$ 0.7	$ 3.2	2.0
Income tax benefit from stock-based compensation	0.3	$ 0.1	0.5	$ 0.3
Unrecognized stock-based compensation expense	$ 8.4		$ 8.4			$ 8.4
Stock-based compensation expenses, recognition period			2 years 6 months 7 days
RLI Corp. Long-Term Incentive Plan (LTIP 2010 and LTIP 2015) \| Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
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