BANNER CORPORATION (PARENT COMPANY ONLY) (Statements of Cash Flows) (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
OPERATING ACTIVITIES:
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
Adjustments to reconcile net income to net cash provided from (used by) operating activities:
Decrease (increase) in deferred taxes													(3,498)	56,267	7,883
Net change in valuation of financial instruments carried at fair value													(3,775)	2,844	2,620
Equity-based compensation													6,554	5,965	4,305
(Increase) decrease in other assets													(15,861)	2,546	3,759
Increase (decrease) in other liabilities													17,322	(179)	(6,664)
Net cash provided from (used by) operating activities													30,775	346,702	(75,818)
INVESTING ACTIVITIES:
Net cash (used by) provided from investing activities													(976,218)	(393,287)	146,447
FINANCING ACTIVITIES:
Payments Related to Tax Withholding for Share-based Compensation													(1,554)	(1,630)	(870)
Payments for Repurchase of Common Stock													34,401	31,045	50,772
Cash dividends paid													(59,280)	(65,759)	(28,282)
Net cash provided from (used by) financing activities													956,439	60,066	(84,827)
NET CHANGE IN CASH AND CASH EQUIVALENTS													10,996	13,481	(14,198)
CASH AND DUE FROM BANKS, BEGINNING OF YEAR				199,624									199,624
CASH AND DUE FROM BANKS, END OF YEAR	231,029				199,624								231,029	199,624
Banner Corporation
OPERATING ACTIVITIES:
Net income													136,515	60,776	85,385
Adjustments to reconcile net income to net cash provided from (used by) operating activities:
Equity in undistributed income of subsidiaries													(72,419)	(27,477)	(40,852)
Decrease (increase) in deferred taxes													150	(1,442)	(702)
Net change in valuation of financial instruments carried at fair value													0	3,507	2,720
Equity-based compensation													6,554	5,965	4,305
(Increase) decrease in other assets													(19,268)	10,684	7,332
Increase (decrease) in other liabilities													201	69	(202)
Net cash provided from (used by) operating activities													51,733	52,082	57,986
INVESTING ACTIVITIES:
Funds transferred to deferred compensation trust													(27)	(29)	(26)
SEC Schedule, 12-04, Cash Dividends Paid to Registrant, Consolidated Subsidiaries													37,000	5,000	50,000
Acquisitions													329	0	0
Net cash (used by) provided from investing activities													36,644	4,971	49,974
FINANCING ACTIVITIES:
Payments Related to Tax Withholding for Share-based Compensation													(1,554)	(1,630)	(870)
Payments for Repurchase of Common Stock													34,401	31,045	50,772
Cash dividends paid													(59,280)	(65,759)	(28,282)
Net cash provided from (used by) financing activities													(95,235)	(98,434)	(79,924)
NET CHANGE IN CASH AND CASH EQUIVALENTS													(6,858)	(41,381)	28,036
CASH AND DUE FROM BANKS, BEGINNING OF YEAR				$ 44,887				$ 86,268				$ 58,232	44,887	86,268	58,232
CASH AND DUE FROM BANKS, END OF YEAR	$ 38,029				$ 44,887				$ 86,268				$ 38,029	$ 44,887	$ 86,268