Document and Entity Information Document - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Entity Information [Line Items]
Entity Registrant Name	Banner Corporation
Document Type	10-Q
Document Period End Date	Sep. 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 Small Business	false
Entity Emerging Growth Company	false
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Voting Common Stock [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		32,324,789
Nonvoting Common Stock [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		74,933