LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Risk-Rated and Non-Risk Rated Loans by Grade and Other Characteristic) (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]
Total loans		$ 4,372,594	$ 3,833,820	$ 3,806,695
Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		1,697,564	1,403,725	1,400,617
Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		198,874	167,524	183,944
Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		493,800	411,006	381,507
Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		812,070	723,964	728,088
Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		242,556	238,499	240,048
One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		536,325	539,894	527,271
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		391,405	349,208	$ 345,220
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	4,277,181	3,737,037
Pass (Risk Ratings 1-5) [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	1,667,522	1,375,885
Pass (Risk Ratings 1-5) [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	195,755	166,712
Pass (Risk Ratings 1-5) [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	478,876	395,356
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	793,518	691,143
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		146,000	115,000
Pass (Risk Ratings 1-5) [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	225,743	234,101
Pass (Risk Ratings 1-5) [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	527,244	527,384
Pass (Risk Ratings 1-5) [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	388,523	346,456
Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		39,488	32,428
Special mention [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		10,108	3,717
Special mention [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Special mention [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Special mention [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		14,189	27,453
Special mention [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		14,960	1,055
Special mention [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		149	63
Special mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		82	140
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		55,913	64,344
Substandard [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		19,934	24,123
Substandard [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		3,119	812
Substandard [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		14,924	15,650
Substandard [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		4,363	5,368
Substandard [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		1,853	3,343
Substandard [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		8,932	12,447
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		2,788	2,601
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		12	11
Doubtful [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Doubtful [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Doubtful [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Doubtful [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Doubtful [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Doubtful [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		12	11
Loss [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0			$ 0
Loss [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0			0
Loss [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0			0
Loss [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0			0
Loss [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0			0
Loss [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0			0
Loss [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0			0
Loss [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0			$ 0
Performing loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		4,349,064	3,817,084
Performing loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		1,691,857	1,402,593
Performing loans [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		198,318	167,524
Performing loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		485,215	409,731
Performing loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		811,085	723,427
Performing loans [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		242,328	236,902
Performing loans [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		530,106	528,965
Performing loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		390,155	347,942
Nonperforming loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[2]	23,530	16,736
Nonperforming loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[2]	5,707	1,132
Nonperforming loans [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[2]	556	0
Nonperforming loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[2]	8,585	1,275
Nonperforming loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[2]	985	537
Nonperforming loans [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[2]	228	1,597
Nonperforming loans [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[2]	6,219	10,929
Nonperforming loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[2]	$ 1,250	$ 1,266

[1] The Pass category includes some performing loans that are part of homogenous pools which are not individually risk-rated. This includes all consumer loans, all one- to four-family residential loans and, as of September 30, 2015 and December 31, 2014, in the commercial business category, $146 million and $115 million, respectively, of credit-scored small business loans. As loans in these pools become non-performing, they are individually risk-rated.

[2] Non-performing loans include non-accrual loans, loans past due greater than 90 days and on accrual status and purchased credit-impaired loans which are included at their unpaid principal balance.