LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Allowance for Credit Losses) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				$ 74,258				$ 76,759					$ 74,258	$ 76,759
Provision for loan losses	0	0	0	0	0	0	0	0	1,000	3,000	4,000	5,000	0	0	13,000
Recoveries													6,993	7,497
Charge-offs													(5,344)	(9,998)
Ending balance	75,907				74,258				76,759				75,907	74,258	76,759
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment	2,966				5,188								2,966	5,188
Allowance collectively evaluated for impairment	72,941				69,070								72,941	69,070
Total allowance for loan losses	75,907				74,258				76,759				75,907	74,258	76,759
Loans individually evaluated for impairment	34,446				60,647								34,446	60,647
Loans collectively evaluated for impairment	3,799,374				3,357,798								3,799,374	3,357,798
Total loans outstanding	3,833,820				3,418,445								3,833,820	3,418,445
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				16,759				15,322					16,759	15,322
Provision for loan losses													1,757	1,639
Recoveries													1,507	2,367
Charge-offs													(1,239)	(2,569)
Ending balance	18,784				16,759								18,784	16,759
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment	728				419								728	419
Allowance collectively evaluated for impairment	18,056				16,340								18,056	16,340
Total allowance for loan losses	18,784				16,759								18,784	16,759
Loans individually evaluated for impairment	7,171				10,877								7,171	10,877
Loans collectively evaluated for impairment	1,396,554				1,184,181								1,396,554	1,184,181
Total loans outstanding	1,403,725				1,195,058								1,403,725	1,195,058
Multifamily Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				5,306				4,506					5,306	4,506
Provision for loan losses													(724)	800
Recoveries													0	0
Charge-offs													(20)	0
Ending balance	4,562				5,306								4,562	5,306
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment	86				1,139								86	1,139
Allowance collectively evaluated for impairment	4,476				4,167								4,476	4,167
Total allowance for loan losses	4,562				5,306								4,562	5,306
Loans individually evaluated for impairment	786				5,744								786	5,744
Loans collectively evaluated for impairment	166,738				131,409								166,738	131,409
Total loans outstanding	167,524				137,153								167,524	137,153
Construction and Land [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				17,640				14,991					17,640	14,991
Provision for loan losses													6,336	2,195
Recoveries													1,776	2,275
Charge-offs													(207)	(1,821)
Ending balance	25,545				17,640								25,545	17,640
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment	986				1,762								986	1,762
Allowance collectively evaluated for impairment	24,559				15,878								24,559	15,878
Total allowance for loan losses	25,545				17,640								25,545	17,640
Loans individually evaluated for impairment	6,477				12,118								6,477	12,118
Loans collectively evaluated for impairment	404,529				339,140								404,529	339,140
Total loans outstanding	411,006				351,258								411,006	351,258
Commercial business [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				11,773				9,957					11,773	9,957
Provision for loan losses													626	1,925
Recoveries													988	1,673
Charge-offs													(1,344)	(1,782)
Ending balance	12,043				11,773								12,043	11,773
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment	82				222								82	222
Allowance collectively evaluated for impairment	11,961				11,551								11,961	11,551
Total allowance for loan losses	12,043				11,773								12,043	11,773
Loans individually evaluated for impairment	739				1,298								739	1,298
Loans collectively evaluated for impairment	723,225				680,871								723,225	680,871
Total loans outstanding	723,964				682,169								723,964	682,169
Agricultural Business [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				2,841				2,295					2,841	2,295
Provision for loan losses													(417)	97
Recoveries													1,576	697
Charge-offs													(179)	(248)
Ending balance	3,821				2,841								3,821	2,841
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment	0				0								0	0
Allowance collectively evaluated for impairment	3,821				2,841								3,821	2,841
Total allowance for loan losses	3,821				2,841								3,821	2,841
Loans individually evaluated for impairment	744				0								744	0
Loans collectively evaluated for impairment	237,755				228,291								237,755	228,291
Total loans outstanding	238,499				228,291								238,499	228,291
One- to four-family residential [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				11,486				16,475					11,486	16,475
Provision for loan losses													(5,772)	(2,995)
Recoveries													618	145
Charge-offs													(885)	(2,139)
Ending balance	5,447				11,486								5,447	11,486
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment	1,014				1,579								1,014	1,579
Allowance collectively evaluated for impairment	4,433				9,907								4,433	9,907
Total allowance for loan losses	5,447				11,486								5,447	11,486
Loans individually evaluated for impairment	17,848				29,834								17,848	29,834
Loans collectively evaluated for impairment	522,046				499,660								522,046	499,660
Total loans outstanding	539,894				529,494								539,894	529,494
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				1,335				1,348					1,335	1,348
Provision for loan losses													90	1,086
Recoveries													528	340
Charge-offs													(1,470)	(1,439)
Ending balance	483				1,335								483	1,335
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment	70				67								70	67
Allowance collectively evaluated for impairment	413				1,268								413	1,268
Total allowance for loan losses	483				1,335								483	1,335
Loans individually evaluated for impairment	681				776								681	776
Loans collectively evaluated for impairment	348,527				294,246								348,527	294,246
Total loans outstanding	349,208				295,022								349,208	295,022
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				7,118				11,865					7,118	11,865
Provision for loan losses													(1,896)	(4,747)
Recoveries													0	0
Charge-offs													0	0
Ending balance	5,222				7,118								5,222	7,118
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment	0				0								0	0
Allowance collectively evaluated for impairment	5,222				7,118								5,222	7,118
Total allowance for loan losses	5,222				7,118								5,222	7,118
Loans individually evaluated for impairment	0				0								0	0
Loans collectively evaluated for impairment	0				0								0	0
Total loans outstanding	$ 0				$ 0								$ 0	$ 0