LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Geographic Type) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 3,833,820	$ 3,418,445
Percent of total loans	100.00%	100.00%
Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,358,106
Percent of total loans	61.50%
Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	869,649
Percent of total loans	22.70%
Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	319,154
Percent of total loans	8.30%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	286,911
Percent of total loans	7.50%
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	546,783	502,601
Percent of total loans	14.30%	14.70%
Commerical real estate - owner-occupied [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	383,950
Commerical real estate - owner-occupied [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	86,937
Commerical real estate - owner-occupied [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	56,348
Commerical real estate - owner-occupied [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	19,548
Commerical real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	856,942	692,457
Percent of total loans	22.30%	20.20%
Commerical real estate - investment properties [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	523,806
Commerical real estate - investment properties [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	124,604
Commerical real estate - investment properties [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	60,053
Commerical real estate - investment properties [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	148,479
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	167,524	137,153
Percent of total loans	4.40%	4.00%
Multifamily real estate [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	116,793
Multifamily real estate [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	35,527
Multifamily real estate [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	14,759
Multifamily real estate [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	445
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	17,337	12,168
Percent of total loans	0.40%	0.40%
Commercial construction [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	15,599
Commercial construction [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0
Commercial construction [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,738
Commercial construction [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	60,193	52,081
Percent of total loans	1.60%	1.50%
Multifamily construction [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	50,931
Multifamily construction [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	8,850
Multifamily construction [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	412
Multifamily construction [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0
One-to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	219,889	200,864
Percent of total loans	5.70%	5.90%
One-to four-family construction [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	129,499
One-to four-family construction [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	88,468
One-to four-family construction [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,922
One-to four-family construction [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	102,435	75,695
Percent of total loans	2.70%	2.20%
Land and land development - residential [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	56,675
Land and land development - residential [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	44,707
Land and land development - residential [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,053
Land and land development - residential [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	11,152	10,450
Percent of total loans	0.30%	0.30%
Land and land development - commercial [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	5,781
Land and land development - commercial [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,529
Land and land development - commercial [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,842
Land and land development - commercial [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	723,964	682,169
Percent of total loans	18.90%	20.00%
Commercial business [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	397,103
Commercial business [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	125,235
Commercial business [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	85,580
Commercial business [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	116,046
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	238,499	228,291
Percent of total loans	6.20%	6.70%
Agricultural business, including secured by farmland [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	119,617
Agricultural business, including secured by farmland [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	69,843
Agricultural business, including secured by farmland [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	48,997
Agricultural business, including secured by farmland [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	42
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	539,894	529,494
Percent of total loans	14.10%	15.50%
One- to four-family residential [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	341,944
One- to four-family residential [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	172,974
One- to four-family residential [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	24,223
One- to four-family residential [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	753
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	222,205	173,188
Percent of total loans	5.80%	5.10%
Consumer secured by one- to four-family [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	136,888
Consumer secured by one- to four-family [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	69,172
Consumer secured by one- to four-family [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	14,984
Consumer secured by one- to four-family [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,161
Consumer - other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	127,003	121,834
Percent of total loans	3.30%	3.50%
Consumer - other [Member] | Washington [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	79,520
Consumer - other [Member] | Oregon [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	40,803
Consumer - other [Member] | Idaho [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	6,243
Consumer - other [Member] | Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 437