Basis of Presentation and Summary of Significant Accounting Policies - Adopted Accounting Standards (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Sep. 30, 2023
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in unconsolidated investees	$ 58,275	$ 56,110
Other Investments	47,922	59,730
Retained earnings	1,446,341	$ 1,598,875
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity investments in tax credit structures, unfunded commitment	7,000
Retained earnings	$ (1,000)
Accounting Standards Update [Extensible Enumeration]	rli:AccountingStandardsUpdate202302Member
Cumulative Effect, Period of Adoption, Adjustment | Other Invested Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Investments	$ 11,000