REGULATORY CAPITAL REQUIREMENTS (Details) $ in Thousands	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Total capital to risk-weighted assets:
Actual	$ 1,904,533	$ 1,769,064
Actual, Ratio	0.1458	0.1404
Minimum for Capital Adequacy Purposes	$ 1,045,181	$ 1,008,232
Minimum for Capital Adequacy Purposes, Ratio	0.0800	0.0800
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 1,306,476	$ 1,260,290
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.1000	0.1000
Tier 1 capital to risk-weighted assets:
Actual	$ 1,650,872	$ 1,528,694
Actual, Ratio	0.1264	0.1213
Minimum for Capital Adequacy Purposes	$ 783,886	$ 756,174
Minimum for Capital Adequacy Purposes, Ratio	0.0600	0.0600
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 783,886	$ 756,174
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.0600	0.0600
Tier 1 leverage capital to average assets:
Actual	$ 1,650,872	$ 1,528,694
Actual, Ratio	0.1056	0.0945
Minimum for Capital Adequacy Purposes	$ 625,387	$ 647,345
Minimum for Capital Adequacy Purposes, Ratio	0.0400	0.0400
Tier 1 common equity to risk-weighted assets:
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual	$ 1,564,372	$ 1,442,194
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Actual	0.1197	0.1144
Banking Regulation, Common Equity Tier One Risk-Based Capital, Capital Adequacy, Minimum	$ 587,914	$ 567,130
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Capital Adequacy, Minimum	0.0450	0.0450
Banner Bank [Member]
Total capital to risk-weighted assets:
Actual	$ 1,789,371	$ 1,684,766
Actual, Ratio	0.1369	0.1338
Minimum for Capital Adequacy Purposes	$ 1,045,273	$ 1,007,325
Minimum for Capital Adequacy Purposes, Ratio	0.0800	0.0800
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 1,306,592	$ 1,259,156
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.1000	0.1000
Tier 1 capital to risk-weighted assets:
Actual	$ 1,635,710	$ 1,544,396
Actual, Ratio	0.1252	0.1227
Minimum for Capital Adequacy Purposes	$ 783,955	$ 755,494
Minimum for Capital Adequacy Purposes, Ratio	0.0600	0.0600
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 1,045,273	$ 1,007,325
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.0800	0.0800
Tier 1 leverage capital to average assets:
Actual	$ 1,635,710	$ 1,544,396
Actual, Ratio	0.1046	0.0955
Minimum for Capital Adequacy Purposes	$ 625,298	$ 646,935
Minimum for Capital Adequacy Purposes, Ratio	0.0400	0.0400
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 781,622	$ 808,668
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	0.0500	0.0500
Tier 1 common equity to risk-weighted assets:
Banking Regulation, Common Equity Tier One Risk-Based Capital, Actual	$ 1,635,710	$ 1,544,396
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Actual	0.1252	0.1227
Banking Regulation, Common Equity Tier One Risk-Based Capital, Capital Adequacy, Minimum	$ 587,966	$ 566,620
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Capital Adequacy, Minimum	0.0450	0.0450
Banking Regulation, Common Equity Tier One Risk-Based Capital, Well Capitalized, Minimum	$ 849,285	$ 818,452
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Well Capitalized, Minimum	0.0650	0.0650