LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				$ 78,008				$ 75,907					$ 78,008	$ 75,907
Provision for loan losses	$ 2,030	$ 2,000	$ 2,000	0	$ 0	$ 0	$ 0	0	$ 0	$ 0	$ 0	$ 0	6,030	0	$ 0
Recoveries													6,698	6,960
Charge-offs													(4,739)	(4,859)
Ending balance	85,997				78,008				75,907				85,997	78,008	75,907
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																$ 1,420	$ 1,920
Allowance collectively evaluated for impairment																84,544	76,033
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																	55
Allowance for loan losses	85,997			78,008	78,008			75,907	75,907				78,008	75,907	75,907	85,997	78,008
Loans individually evaluated for impairment																30,382	28,816
Loans collectively evaluated for impairment																7,388,444	7,227,088
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																32,322	58,600
Loans receivable																7,451,148	7,314,504
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				20,716				18,784					20,716	18,784
Provision for loan losses													441	1,177
Recoveries													582	819
Charge-offs													(746)	(64)
Ending balance	20,993				20,716				18,784				20,993	20,716	18,784
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																428	605
Allowance collectively evaluated for impairment																20,565	20,111
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	20,993			20,716	20,716			18,784	18,784				20,716	18,784	18,784	20,993	20,716
Loans individually evaluated for impairment																10,300	8,519
Loans collectively evaluated for impairment																3,305,586	3,043,656
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																23,449	40,985
Loans receivable																3,339,335	3,093,160
Multifamily Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				4,195				4,562					4,195	4,562
Provision for loan losses													(2,835)	(480)
Recoveries													0	113
Charge-offs													0	0
Ending balance	1,360				4,195				4,562				1,360	4,195	4,562
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																64	71
Allowance collectively evaluated for impairment																1,296	4,124
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	1,360			4,195	4,195			4,562	4,562				4,195	4,562	4,562	1,360	4,195
Loans individually evaluated for impairment																349	357
Loans collectively evaluated for impairment																247,662	470,625
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																139	1,994
Loans receivable																248,150	472,976
Construction and Land [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				27,131				25,545					27,131	25,545
Provision for loan losses													5,566	666
Recoveries													2,171	1,811
Charge-offs													(616)	(891)
Ending balance	34,252				27,131				25,545				34,252	27,131	25,545
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																374	418
Allowance collectively evaluated for impairment																33,845	26,713
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																33	0
Allowance for loan losses	34,252			27,131	27,131			25,545	25,545				27,131	25,545	25,545	34,252	27,131
Loans individually evaluated for impairment																4,034	4,669
Loans collectively evaluated for impairment																815,174	564,051
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																3,878	5,650
Loans receivable																823,086	574,370
Commercial business [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				13,856				12,043					13,856	12,043
Provision for loan losses													1,632	1,611
Recoveries													1,993	948
Charge-offs													(948)	(746)
Ending balance	16,533				13,856				12,043				16,533	13,856	12,043
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																69	69
Allowance collectively evaluated for impairment																16,464	13,732
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	55
Allowance for loan losses	16,533			13,856	13,856			12,043	12,043				13,856	12,043	12,043	16,533	13,856
Loans individually evaluated for impairment																2,946	2,223
Loans collectively evaluated for impairment																1,201,112	1,198,419
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																3,821	7,302
Loans receivable																1,207,879	1,207,944
Agricultural Business [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				3,645				3,821					3,645	3,821
Provision for loan losses													(170)	(878)
Recoveries													59	1,927
Charge-offs													(567)	(1,225)
Ending balance	2,967				3,645				3,821				2,967	3,645	3,821
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																0	0
Allowance collectively evaluated for impairment																2,967	3,645
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	2,967			3,645	3,645			3,821	3,821				3,645	3,821	3,821	2,967	3,645
Loans individually evaluated for impairment																4,766	544
Loans collectively evaluated for impairment																363,706	374,458
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																684	1,529
Loans receivable																369,156	376,531
One- to four-family residential [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				4,732				5,447					4,732	5,447
Provision for loan losses													(3,402)	(1,068)
Recoveries													1,283	772
Charge-offs													(375)	(419)
Ending balance	2,238				4,732				5,447				2,238	4,732	5,447
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																480	728
Allowance collectively evaluated for impairment																1,758	4,004
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	2,238			4,732	4,732			5,447	5,447				4,732	5,447	5,447	2,238	4,732
Loans individually evaluated for impairment																7,678	12,185
Loans collectively evaluated for impairment																805,125	939,382
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																274	1,066
Loans receivable																813,077	952,633
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				902				483					902	483
Provision for loan losses													4,079	1,363
Recoveries													610	570
Charge-offs													(1,487)	(1,514)
Ending balance	4,104				902				483				4,104	902	483
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																5	29
Allowance collectively evaluated for impairment																4,099	873
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	4,104			902	902			483	483				902	483	483	4,104	902
Loans individually evaluated for impairment																309	319
Loans collectively evaluated for impairment																650,079	636,497
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																77	74
Loans receivable																650,465	636,890
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				2,831				5,222					2,831	5,222
Provision for loan losses													719	(2,391)
Recoveries													0	0
Charge-offs													0	0
Ending balance	3,550				2,831				5,222				3,550	2,831	5,222
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																0	0
Allowance collectively evaluated for impairment																3,550	2,831
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	3,550			$ 2,831	$ 2,831			$ 5,222	$ 5,222				2,831	$ 5,222	$ 5,222	3,550	2,831
Loans individually evaluated for impairment																0	0
Loans collectively evaluated for impairment																0	0
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																0	0
Loans receivable																0	$ 0
Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending balance	33												33
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses	$ 33												$ 33			$ 33