Document and Entity Information Document - shares	6 Months Ended
	Jun. 30, 2018	Jul. 31, 2018
Entity Information [Line Items]
Entity Registrant Name	Banner Corporation
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Amendment Flag	false
Entity Central Index Key	0000946673
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Voting Common Stock [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		32,327,979
Nonvoting Common Stock [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		74,933