BANNER CORPORATION (PARENT COMPANY ONLY) (Statements of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
INTEREST INCOME:
Interest-bearing deposits	$ 119,491	$ 116,621	$ 116,695	$ 116,104	$ 117,459	$ 109,078	$ 105,063	$ 99,373	$ 98,264	$ 100,210	$ 99,706	$ 94,854	$ 468,911	$ 430,973	$ 393,034
OTHER INCOME (EXPENSE):
Other income													8,632	7,148	8,985
Other expenses													(28,122)	(26,754)	(27,142)
Income before provision for income taxes	42,083	48,179	50,655	42,215	40,581	45,857	41,643	37,029	41,438	35,960	38,245	35,621	183,132	165,110	151,264
PROVISION FOR INCOME TAXES	8,428	8,602	10,955	8,869	3,053	8,084	9,219	8,239	54,986	10,883	12,791	11,828	36,854	28,595	90,488
NET INCOME	$ 33,655	$ 39,577	$ 39,700	$ 33,346	$ 37,528	$ 37,773	$ 32,424	$ 28,790	$ (13,548)	$ 25,077	$ 25,454	$ 23,793	146,278	136,515	60,776
Banner Corporation
INTEREST INCOME:
Interest-bearing deposits													98	49	62
OTHER INCOME (EXPENSE):
Dividend income from subsidiaries													119,333	72,604	40,570
Equity in undistributed income of subsidiaries													35,134	72,419	27,477
Other income													33	56	53
Net change in valuation of financial instruments carried at fair value													0	0	(3,507)
Interest on other borrowings													(6,574)	(6,136)	(4,752)
Other expenses													(4,045)	(4,761)	(3,291)
Income before provision for income taxes													143,979	134,231	56,612
PROVISION FOR INCOME TAXES													(2,299)	(2,284)	(4,164)
NET INCOME													$ 146,278	$ 136,515	$ 60,776