SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Accounting Standards (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2016	Dec. 31, 2015
Maximum period of disclosure under ASU 2015-09	10 years
Accounting Standards Update 2015-03 \| Restatement Adjustment
New accounting pronouncement or change in accounting principle effect of change in other assets and long term debt		$ 1.1

Amount of effect the new accounting pronouncement on other assets and long term debt in the statement of financial position as of the beginning of the earliest period presented.

No definition available.

Name:	rli_NewAccountingPronouncementOrChangeInAccountingPrincipleEffectOfChangeOnOtherAssetsAndLongTermDebt
Namespace Prefix:	rli_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration