LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				$ 89,028				$ 85,997				$ 78,008	$ 89,028	$ 85,997	$ 78,008
Provision for loan losses	$ 2,500	$ 2,000	$ 2,000	2,000	$ 2,000	$ 2,000	$ 2,000	2,000	$ 2,030	$ 2,000	$ 2,000	0	8,500	8,000	6,030
Recoveries													4,088	3,731	6,698
Charge-offs													(5,131)	(8,700)	(4,739)
Ending balance	96,485				89,028				85,997				96,485	89,028	85,997
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																$ 416	$ 782
Allowance collectively evaluated for impairment																95,986	88,085
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																	161
Allowance for loan losses	96,485			89,028	89,028			85,997	85,997			78,008	89,028	85,997	78,008	96,485	89,028
Loans individually evaluated for impairment																20,075	32,512
Loans collectively evaluated for impairment																8,650,107	7,545,062
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																14,413	21,310
Loans receivable																8,684,595	7,598,884
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				22,824				20,993				20,716	22,824	20,993	20,716
Provision for loan losses													3,063	2,639	441
Recoveries													1,646	372	582
Charge-offs													(401)	(1,180)	(746)
Ending balance	27,132				22,824				20,993				27,132	22,824	20,993
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																246	281
Allowance collectively evaluated for impairment																26,886	22,543
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	27,132			22,824	22,824			20,993	20,993			20,716	22,824	20,993	20,716	27,132	22,824
Loans individually evaluated for impairment																8,625	16,017
Loans collectively evaluated for impairment																3,540,538	3,190,921
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																11,993	14,848
Loans receivable																3,561,156	3,221,786
Multifamily Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				1,633				1,360				4,195	1,633	1,360	4,195
Provision for loan losses													2,185	262	(2,835)
Recoveries													0	11	0
Charge-offs													0	0	0
Ending balance	3,818				1,633				1,360				3,818	1,633	1,360
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																0	0
Allowance collectively evaluated for impairment																3,818	1,633
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	3,818			1,633	1,633			1,360	1,360			4,195	1,633	1,360	4,195	3,818	1,633
Loans individually evaluated for impairment																0	0
Loans collectively evaluated for impairment																368,698	314,019
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																138	169
Loans receivable																368,836	314,188
Construction and Land [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				27,568				34,252				27,131	27,568	34,252	27,131
Provision for loan losses													(2,860)	(7,921)	5,566
Recoveries													213	1,237	2,171
Charge-offs													(479)	0	(616)
Ending balance	24,442				27,568				34,252				24,442	27,568	34,252
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																0	0
Allowance collectively evaluated for impairment																24,442	27,567
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	1
Allowance for loan losses	24,442			27,568	27,568			34,252	34,252			27,131	27,568	34,252	27,131	24,442	27,568
Loans individually evaluated for impairment																3,096	750
Loans collectively evaluated for impairment																1,104,271	903,408
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																137	3,365
Loans receivable																1,107,504	907,523
Commercial business [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				18,311				16,533				13,856	18,311	16,533	13,856
Provision for loan losses													2,129	4,355	1,632
Recoveries													1,049	1,226	1,993
Charge-offs													(2,051)	(3,803)	(948)
Ending balance	19,438				18,311				16,533				19,438	18,311	16,533
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																16	50
Allowance collectively evaluated for impairment																19,399	18,214
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																23	47
Allowance for loan losses	19,438			18,311	18,311			16,533	16,533			13,856	18,311	16,533	13,856	19,438	18,311
Loans individually evaluated for impairment																391	1,812
Loans collectively evaluated for impairment																1,482,195	1,275,923
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																1,028	2,159
Loans receivable																1,483,614	1,279,894
Agricultural Business [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				4,053				2,967				3,645	4,053	2,967	3,645
Provision for loan losses													417	3,326	(170)
Recoveries													64	134	59
Charge-offs													(756)	(2,374)	(567)
Ending balance	3,778				4,053				2,967				3,778	4,053	2,967
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																96	264
Allowance collectively evaluated for impairment																3,622	3,676
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																60	113
Allowance for loan losses	3,778			4,053	4,053			2,967	2,967			3,645	4,053	2,967	3,645	3,778	4,053
Loans individually evaluated for impairment																3,298	8,585
Loans collectively evaluated for impairment																401,082	329,238
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																493	565
Loans receivable																404,873	338,388
One- to four-family residential [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				2,055				2,238				4,732	2,055	2,238	4,732
Provision for loan losses													1,952	(415)	(3,402)
Recoveries													750	270	1,283
Charge-offs													(43)	(38)	(375)
Ending balance	4,714				2,055				2,238				4,714	2,055	2,238
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																51	178
Allowance collectively evaluated for impairment																4,663	1,877
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	4,714			2,055	2,055			2,238	2,238			4,732	2,055	2,238	4,732	4,714	2,055
Loans individually evaluated for impairment																4,469	5,136
Loans collectively evaluated for impairment																969,046	843,017
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																101	136
Loans receivable																973,616	848,289
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				3,866				4,104				902	3,866	4,104	902
Provision for loan losses													5,141	586	4,079
Recoveries													366	481	610
Charge-offs													(1,401)	(1,305)	(1,487)
Ending balance	7,972				3,866				4,104				7,972	3,866	4,104
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																7	9
Allowance collectively evaluated for impairment																7,965	3,857
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	7,972			3,866	3,866			4,104	4,104			902	3,866	4,104	902	7,972	3,866
Loans individually evaluated for impairment																196	212
Loans collectively evaluated for impairment																784,277	688,536
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net																523	68
Loans receivable																784,996	688,816
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				8,718				3,550				2,831	8,718	3,550	2,831
Provision for loan losses													(3,527)	5,168	719
Recoveries													0	0	0
Charge-offs													0	0	0
Ending balance	5,191				8,718				3,550				5,191	8,718	3,550
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance individually evaluated for impairment																0	0
Allowance collectively evaluated for impairment																5,191	8,718
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses																0	0
Allowance for loan losses	$ 5,191			8,718	8,718			$ 3,550	$ 3,550			$ 2,831	8,718	3,550	$ 2,831	5,191	8,718
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
Financial Asset Acquired with Credit Deterioration [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				83									83
Ending balance					83									83
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses				$ 83	$ 83								$ 83	$ 83			$ 83