Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting
Net premiums earned	$ 1,614,346	$ 1,526,406	$ 1,294,306
Casualty Segment
Segment Reporting
Net premiums earned	953,981	852,837	758,346
Casualty Segment \| Commercial excess and personal umbrella
Segment Reporting
Net premiums earned	447,361	354,847	286,178
Casualty Segment \| Commercial transportation
Segment Reporting
Net premiums earned	123,413	120,650	103,719
Casualty Segment \| General liability
Segment Reporting
Net premiums earned	110,891	104,423	103,066
Casualty Segment \| Professional services
Segment Reporting
Net premiums earned	108,090	103,794	99,596
Casualty Segment \| Small commercial
Segment Reporting
Net premiums earned	79,064	78,308	72,920
Casualty Segment \| Executive products
Segment Reporting
Net premiums earned	22,942	23,555	24,687
Casualty Segment \| Other casualty
Segment Reporting
Net premiums earned	62,220	67,260	68,180
Property Segment
Segment Reporting
Net premiums earned	512,404	531,384	401,530
Property Segment \| Commercial property
Segment Reporting
Net premiums earned	301,659	345,554	244,798
Property Segment \| Marine
Segment Reporting
Net premiums earned	158,904	145,706	129,428
Property Segment \| Other property
Segment Reporting
Net premiums earned	51,841	40,124	27,304
Surety Segment
Segment Reporting
Net premiums earned	147,961	142,185	134,430
Surety Segment \| Transactional
Segment Reporting
Net premiums earned	52,418	49,460	47,983
Surety Segment \| Commercial
Segment Reporting
Net premiums earned	50,690	48,533	49,707
Surety Segment \| Contract
Segment Reporting
Net premiums earned	$ 44,853	$ 44,192	$ 36,740

RLI CORP 10-K