REGULATORY CAPITAL REQUIREMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Total capital to risk-weighted assets:
Actual	$ 1,302,239	$ 1,214,631
Actual, Ratio	13.12%	13.81%
Minimum for Capital Adequacy Purposes	$ 794,072	$ 703,508
Minimum for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 992,590	$ 879,385
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	$ 1,203,155	$ 1,123,154
Actual, Ratio	12.12%	12.77%
Minimum for Capital Adequacy Purposes	$ 595,554	$ 527,631
Minimum for Capital Adequacy Purposes, Ratio	6.00%	6.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 595,554	$ 527,631
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	6.00%	6.00%
Common Equity Tier One Capital	$ 1,067,155
Common Equity Tier One Capital Ratio	10.75%
Tier 1 common equity to risk-weighted assets:
Actual		$ 994,080
Actual, Ratio		11.30%
Common Equity Tier One Capital Required for Capital Adequacy	$ 446,665	$ 395,723
Minimum for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Tier 1 leverage capital to average assets:
Actual	$ 1,203,155	$ 1,123,154
Actual, Ratio	10.98%	11.34%
Minimum for Capital Adequacy Purposes	$ 438,379	$ 396,313
Minimum for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Banner Bank [Member]
Total capital to risk-weighted assets:
Actual	$ 1,217,173	$ 1,102,195
Actual, Ratio	12.50%	12.83%
Minimum for Capital Adequacy Purposes	$ 778,766	$ 687,266
Minimum for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 973,457	$ 859,083
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	$ 1,120,523	$ 1,013,079
Actual, Ratio	11.51%	11.79%
Minimum for Capital Adequacy Purposes	$ 584,074	$ 515,450
Minimum for Capital Adequacy Purposes, Ratio	6.00%	6.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 778,766	$ 687,266
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Common Equity Tier One Capital	$ 1,120,523
Common Equity Tier One Capital Ratio	11.51%
Tier 1 common equity to risk-weighted assets:
Actual		$ 1,013,079
Actual, Ratio		11.79%
Common Equity Tier One Capital Required for Capital Adequacy	$ 438,056	$ 386,587
Minimum for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Common Equity Tier One Capital Required to be Well-Capitalized	$ 632,747	$ 558,404
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	$ 1,120,523	$ 1,013,079
Actual, Ratio	10.50%	10.53%
Minimum for Capital Adequacy Purposes	$ 426,799	$ 384,920
Minimum for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 533,498	$ 481,150
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	$ 34,567	$ 32,122
Actual, Ratio	18.26%	16.39%
Minimum for Capital Adequacy Purposes	$ 15,142	$ 15,681
Minimum for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 18,928	$ 19,602
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	$ 32,200	$ 29,761
Actual, Ratio	17.01%	15.18%
Minimum for Capital Adequacy Purposes	$ 11,357	$ 11,761
Minimum for Capital Adequacy Purposes, Ratio	6.00%	6.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 15,142	$ 15,681
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Common Equity Tier One Capital	$ 32,200
Common Equity Tier One Capital Ratio	17.01%
Tier 1 common equity to risk-weighted assets:
Actual		$ 29,761
Actual, Ratio		15.18%
Common Equity Tier One Capital Required for Capital Adequacy	$ 8,518	$ 8,821
Minimum for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Common Equity Tier One Capital Required to be Well-Capitalized	$ 12,303	$ 12,741
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	$ 32,200	$ 29,761
Actual, Ratio	11.16%	10.65%
Minimum for Capital Adequacy Purposes	$ 11,543	$ 11,183
Minimum for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 14,428	$ 13,979
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%