Submission	May 21, 2026
Submission [Line Items]
Central Index Key	0000860413
Registrant Name	FIRST INTERSTATE BANCSYSTEM INC
Form Type	S-3
Submission Type	S-3ASR
Fee Exhibit Type	EX-FILING FEES

A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Exchange Act
-Number 240
-Section 12
-Subsection b-2

Name:	dei_EntityCentralIndexKey
Namespace Prefix:	dei_
Data Type:	dei:centralIndexKeyItemType
Balance Type:	na
Period Type:	duration