Summary of Significant Accounting Policies - Adopted Accounting Standards (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Jun. 30, 2023
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in unconsolidated investees	$ 58,275	$ 55,250
Other Investments	47,922	60,907
Retained earnings	$ 1,446,341	1,597,660
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible Enumeration]	rli:AccountingStandardsUpdate202302Member
Historic tax credit partnerships (HTC)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in unconsolidated investees	$ 11,000
Other Investments		15,000
Equity investments in tax credit structures, unfunded commitment		$ 4,000
Historic tax credit partnerships (HTC) | 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)
Historic tax credit partnerships (HTC) | Cumulative Effect, Period of Adoption, Adjustment | Other Invested Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Investments	$ 11,000