BANNER CORPORATION (PARENT COMPANY ONLY) (Statements of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
INTEREST INCOME:
Interest-bearing deposits	$ 121,437	$ 121,026	$ 119,580	$ 119,258	$ 119,491	$ 116,623	$ 116,697	$ 116,108	$ 117,462	$ 109,090	$ 105,063	$ 99,373	$ 481,301	$ 468,919	$ 430,988
OTHER INCOME (EXPENSE):
Other income													6,323	8,624	7,133
Interest on other borrowings													(603)	(330)	(245)
Other expenses													(22,712)	(28,122)	(26,754)
Income before provision for income taxes	48,788	44,040	28,135	21,490	42,083	48,179	50,655	42,215	40,581	45,857	41,643	37,029	142,453	183,132	165,110
PROVISION FOR INCOME TAXES	9,831	7,492	4,594	4,608	8,428	8,602	10,955	8,869	3,053	8,084	9,219	8,239	26,525	36,854	28,595
NET INCOME	$ 38,957	$ 36,548	$ 23,541	$ 16,882	$ 33,655	$ 39,577	$ 39,700	$ 33,346	$ 37,528	$ 37,773	$ 32,424	$ 28,790	115,928	146,278	136,515
Banner Corporation
INTEREST INCOME:
Interest-bearing deposits													112	98	49
OTHER INCOME (EXPENSE):
Dividend income from subsidiaries													87,748	119,333	72,604
Equity in undistributed income of subsidiaries													36,401	35,134	72,419
Other income													62	33	56
Interest on other borrowings													(7,204)	(6,574)	(6,136)
Other expenses													(3,530)	(4,045)	(4,761)
Income before provision for income taxes													113,589	143,979	134,231
PROVISION FOR INCOME TAXES													(2,339)	(2,299)	(2,284)
NET INCOME													$ 115,928	$ 146,278	$ 136,515