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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2024
Feb. 13, 2025
Jun. 30, 2024
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Document Period End Date Dec. 31, 2024    
Entity File Number 001-09463    
Entity Registrant Name RLI CORP    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 37-0889946    
Entity Address, Address Line One 9025 North Lindbergh Drive    
Entity Address, City or Town Peoria    
Entity Address, State or Province IL    
Entity Address, Postal Zip Code 61615    
City Area Code 309    
Local Phone Number 692-1000    
Title of 12(b) Security Common Stock $0.01 par value    
Trading Symbol RLI    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 6,006,400,931
Entity Common Stock, Shares Outstanding   91,764,132  
Documents Incorporated by Reference

Portions of the registrant’s definitive proxy statement for the 2025 annual meeting of shareholders are incorporated herein by reference into Part III of this document.

    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2024    
Document Fiscal Period Focus FY    
Entity Central Index Key 0000084246    
Amendment Flag false    
Auditor Name Deloitte & Touche LLP    
Auditor Location Chicago, Illinois    
Auditor Firm ID 34