Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Jan. 31, 2019	Jun. 30, 2018
Document Information [Line Items]
Entity Registrant Name	Banner Corporation
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Entity Central Index Key	0000946673
Current Fiscal Year End Date	--12-31
Entity Public Float			$ 1,914,245,405
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	FY
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Voting Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		35,106,227
Nonvoting Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		74,933