Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 07, 2018	Jun. 30, 2017
Document and Entity Information
Entity Registrant Name	RLI CORP
Entity Central Index Key	0000084246
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,125,028,624
Entity Common Stock, Shares Outstanding		44,237,951
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY