LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Risk-Rated and Non-Risk Rated Loans by Grade and Other Characteristic) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	$ 4,114,818		$ 3,833,820		$ 3,522,912
Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,564,224		1,403,725		1,251,099
Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	208,687		167,524		153,003
Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	430,963		411,006		378,774
Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	776,579		723,964		716,546
Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	208,635		238,499		208,817
One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	552,423		539,894		517,621
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	373,307		349,208		297,052
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	4,001,645	[1]	3,737,037	[1]
Pass (Risk Ratings 1-5) [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,533,181	[1]	1,375,885	[1]
Pass (Risk Ratings 1-5) [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	206,791	[1]	166,712	[1]
Pass (Risk Ratings 1-5) [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	410,039	[1]	395,356	[1]
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	742,040	[1]	691,143	[1]
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	120,000		115,000
Pass (Risk Ratings 1-5) [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	199,436	[1]	234,101	[1]
Pass (Risk Ratings 1-5) [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	540,693	[1]	527,384	[1]
Pass (Risk Ratings 1-5) [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	369,465	[1]	346,456	[1]
Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	33,660		32,428
Special mention [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	5,156		3,717
Special mention [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Special mention [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Special mention [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	26,776		27,453
Special mention [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	818		1,055
Special mention [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	691		63
Special mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	219		140
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	79,501		64,344
Substandard [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	25,887		24,123
Substandard [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,896		812
Substandard [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	20,924		15,650
Substandard [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	7,763		5,368
Substandard [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	8,381		3,343
Substandard [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	11,039		12,447
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	3,611		2,601
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	12		11
Doubtful [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Doubtful [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Doubtful [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Doubtful [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Doubtful [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Doubtful [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	0		0
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	12		11
Performing loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	4,090,083		3,817,083
Performing loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,560,083		1,402,328
Performing loans [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	208,109		167,524
Performing loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	423,441		409,731
Performing loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	776,161		723,427
Performing loans [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	207,069		236,902
Performing loans [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	543,764		529,292
Performing loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	371,456		347,879
Nonperforming loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	24,735	[2]	16,737	[2]
Nonperforming loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	4,141	[2]	1,397	[2]
Nonperforming loans [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	578	[2]	0	[2]
Nonperforming loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	7,522	[2]	1,275	[2]
Nonperforming loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	418	[2]	537	[2]
Nonperforming loans [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	1,566	[2]	1,597	[2]
Nonperforming loans [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	8,659	[2]	10,602	[2]
Nonperforming loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans outstanding	$ 1,851	[2]	$ 1,329	[2]

[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 March 31, 2015 and December 31, 2014, in the commercial business category, $120 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 and loans past due greater than 90 days and on accrual status.