BANNER CORPORATION (PARENT COMPANY ONLY) (Statements of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
INTEREST INCOME:
Interest-bearing deposits	$ 117,459	$ 109,078	$ 105,063	$ 99,373	$ 98,264	$ 100,210	$ 99,706	$ 94,854	$ 97,171	$ 93,708	$ 93,148	$ 91,043	$ 430,973	$ 393,034	$ 375,069
OTHER INCOME (EXPENSE):
Other income													7,148	8,985	6,001
Other expenses													(26,754)	(27,142)	(30,131)
Income before provision for income taxes	40,581	45,857	41,643	37,029	41,438	35,960	38,245	35,621	34,747	36,128	31,798	26,968	165,110	151,264	129,640
PROVISION FOR INCOME TAXES	3,053	8,084	9,219	8,239	54,986	10,883	12,791	11,828	11,943	12,277	10,841	9,194	28,595	90,488	44,255
NET INCOME	$ 37,528	$ 37,773	$ 32,424	$ 28,790	$ (13,548)	$ 25,077	$ 25,454	$ 23,793	$ 22,804	$ 23,851	$ 20,957	$ 17,774	136,515	60,776	85,385
Banner Corporation
INTEREST INCOME:
Interest-bearing deposits													49	62	127
OTHER INCOME (EXPENSE):
Dividend income from subsidiaries													72,604	40,570	50,971
Equity in undistributed income of subsidiaries													72,419	27,477	40,852
Other income													56	53	60
Net change in valuation of financial instruments carried at fair value													0	(3,507)	(2,720)
Interest on other borrowings													(6,136)	(4,752)	(4,040)
Other expenses													(4,761)	(3,291)	(3,450)
Income before provision for income taxes													134,231	56,612	81,800
PROVISION FOR INCOME TAXES													(2,284)	(4,164)	(3,585)
NET INCOME													$ 136,515	$ 60,776	$ 85,385