REGULATORY CAPITAL REQUIREMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Total capital to risk-weighted assets:
Actual	$ 1,214,913	$ 1,139,554
Actual, Ratio	13.40%	13.63%
Minimum for Capital Adequacy Purposes	$ 725,566	$ 668,941
Minimum for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 906,957	$ 836,530
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	$ 1,125,267	$ 1,057,597
Actual, Ratio	12.41%	12.65%
Minimum for Capital Adequacy Purposes	$ 544,174	$ 501,706
Minimum for Capital Adequacy Purposes, Ratio	6.00%	6.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 544,174	$ 501,918
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	6.00%	6.00%
Tier 1 common equity to risk-weighted assets:
Actual	$ 1,014,994	$ 1,013,971
Actual, Ratio	11.19%	12.13%
Minimum for Capital Adequacy Purposes	$ 408,131	$ 376,279
Minimum for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Tier 1 leverage capital to average assets:
Actual	$ 1,125,267	$ 1,057,597
Actual, Ratio	11.83%	11.06%
Minimum for Capital Adequacy Purposes	$ 380,519	$ 382,617
Minimum for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Banner Bank [Member]
Total capital to risk-weighted assets:
Actual	$ 1,043,837	$ 1,030,601
Actual, Ratio	11.76%	12.61%
Minimum for Capital Adequacy Purposes	$ 709,882	$ 653,606
Minimum for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 887,352	$ 817,008
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	$ 956,298	$ 950,865
Actual, Ratio	10.78%	11.64%
Minimum for Capital Adequacy Purposes	$ 532,411	$ 490,205
Minimum for Capital Adequacy Purposes, Ratio	6.00%	6.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 709,882	$ 653,606
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Tier 1 common equity to risk-weighted assets:
Actual	$ 956,298	$ 950,865
Actual, Ratio	10.78%	11.64%
Minimum for Capital Adequacy Purposes	$ 399,308	$ 367,653
Minimum for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 576,779	$ 531,055
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Tier 1 leverage capital to average assets:
Actual	$ 956,298	$ 950,865
Actual, Ratio	10.34%	10.23%
Minimum for Capital Adequacy Purposes	$ 369,936	$ 371,807
Minimum for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 462,420	$ 464,759
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	$ 35,207	$ 38,448
Actual, Ratio	18.43%	20.31%
Minimum for Capital Adequacy Purposes	$ 15,281	$ 15,146
Minimum for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 19,101	$ 18,932
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	$ 33,099	$ 36,227
Actual, Ratio	17.33%	19.14%
Minimum for Capital Adequacy Purposes	$ 11,461	$ 11,359
Minimum for Capital Adequacy Purposes, Ratio	6.00%	6.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 15,281	$ 15,146
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Tier 1 common equity to risk-weighted assets:
Actual	$ 33,099	$ 36,227
Actual, Ratio	17.33%	19.14%
Minimum for Capital Adequacy Purposes	$ 8,598	$ 8,520
Minimum for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 12,416	$ 12,306
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Tier 1 leverage capital to average assets:
Actual	$ 33,099	$ 36,227
Actual, Ratio	12.72%	13.38%
Minimum for Capital Adequacy Purposes	$ 10,405	$ 10,826
Minimum for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions	$ 13,006	$ 13,533
Minimum to be Categorized as Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%