REGULATORY CAPITAL REQUIREMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Total capital to risk-weighted assets:
Actual	$ 1,386,483	$ 1,302,239
Actual, Ratio	12.93%	13.12%
Minimum for Capital Adequacy Purposes	$ 857,546	$ 794,072
Minimum for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 1,071,933	$ 992,590
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier 1 capital to risk-weighted assets:
Actual	$ 1,283,208	$ 1,203,155
Actual, Ratio	11.97%	12.12%
Minimum for Capital Adequacy Purposes	$ 643,160	$ 595,554
Minimum for Capital Adequacy Purposes, Ratio	6.00%	6.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 643,160	$ 595,554
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	6.00%	6.00%
Common Equity Tier One Capital	$ 1,139,708
Common Equity Tier One Capital Ratio	10.63%
Tier 1 common equity to risk-weighted assets:
Actual		$ 1,067,155
Actual, Ratio		10.75%
Common Equity Tier One Capital Required for Capital Adequacy	$ 482,370	$ 446,665
Minimum for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Tier 1 leverage capital to average assets:
Actual	$ 1,283,208	$ 1,203,155
Actual, Ratio	10.71%	10.98%
Minimum for Capital Adequacy Purposes	$ 479,458	$ 438,379
Minimum for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Banner Bank [Member]
Total capital to risk-weighted assets:
Actual	$ 1,321,580	$ 1,217,173
Actual, Ratio	12.55%	12.50%
Minimum for Capital Adequacy Purposes	$ 842,219	$ 778,766
Minimum for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 1,052,773	$ 973,457
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier 1 capital to risk-weighted assets:
Actual	$ 1,220,811	$ 1,120,523
Actual, Ratio	11.60%	11.51%
Minimum for Capital Adequacy Purposes	$ 631,664	$ 584,074
Minimum for Capital Adequacy Purposes, Ratio	6.00%	6.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 842,219	$ 778,766
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Common Equity Tier One Capital	$ 1,220,811
Common Equity Tier One Capital Ratio	11.60%
Tier 1 common equity to risk-weighted assets:
Actual		$ 1,120,523
Actual, Ratio		11.51%
Common Equity Tier One Capital Required for Capital Adequacy	$ 473,748	$ 438,056
Minimum for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Common Equity Tier One Capital Required to be Well-Capitalized	$ 684,303	$ 632,747
Tier One Risk Based Common Equity Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier 1 leverage capital to average assets:
Actual	$ 1,220,811	$ 1,120,523
Actual, Ratio	10.45%	10.50%
Minimum for Capital Adequacy Purposes	$ 467,330	$ 426,799
Minimum for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 584,163	$ 533,498
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%
Islanders Bank
Total capital to risk-weighted assets:
Actual	$ 37,044	$ 34,567
Actual, Ratio	19.42%	18.26%
Minimum for Capital Adequacy Purposes	$ 15,258	$ 15,142
Minimum for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 19,073	$ 18,928
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier 1 capital to risk-weighted assets:
Actual	$ 34,658	$ 32,200
Actual, Ratio	18.17%	17.01%
Minimum for Capital Adequacy Purposes	$ 11,444	$ 11,357
Minimum for Capital Adequacy Purposes, Ratio	6.00%	6.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 15,258	$ 15,142
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Common Equity Tier One Capital	$ 34,658
Common Equity Tier One Capital Ratio	18.17%
Tier 1 common equity to risk-weighted assets:
Actual		$ 32,200
Actual, Ratio		17.01%
Common Equity Tier One Capital Required for Capital Adequacy	$ 8,583	$ 8,518
Minimum for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Common Equity Tier One Capital Required to be Well-Capitalized	$ 12,397	$ 12,303
Tier One Risk Based Common Equity Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Tier 1 leverage capital to average assets:
Actual	$ 34,658	$ 32,200
Actual, Ratio	11.66%	11.16%
Minimum for Capital Adequacy Purposes	$ 11,887	$ 11,543
Minimum for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 14,859	$ 14,428
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%