LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Allowance for Loan Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Mar. 31, 2017
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			$ 89,028	$ 85,997
Provision for loan losses	$ 2,000	$ 2,000	6,000	6,000
Recoveries	735	776	3,663	3,153
Charge-offs	(1,347)	(2,262)	(3,428)	(6,050)
Allowance for loan losses, Ending balance	95,263	89,100	95,263	89,100
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					$ 457			$ 773
Allowance collectively evaluated for impairment					94,721			88,320
Allowance for purchased credit-impaired loans					85			7
Total allowance for loan losses	95,263	89,100	89,028	85,997	95,263	$ 93,875	$ 89,028	89,100	$ 88,586
Loans individually evaluated for impairment					18,818			33,531
Loans collectively evaluated for impairment					7,790,757			7,717,697
Purchased Credit-Impaired					12,944		21,310	23,221
Total loans					7,822,519		7,598,884	7,774,449
Commercial real estate [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			22,824	20,993
Provision for loan losses	824	(236)	1,144	2,716
Recoveries	12	19	1,580	353
Charge-offs	(102)	(584)	(401)	(631)
Allowance for loan losses, Ending balance	25,147	23,431	25,147	23,431
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					297			263
Allowance collectively evaluated for impairment					24,850			23,168
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	25,147	23,431	22,824	20,993	25,147	24,413	22,824	23,431	24,232
Loans individually evaluated for impairment					8,769			13,866
Loans collectively evaluated for impairment					3,198,018			3,332,724
Purchased Credit-Impaired					8,369			15,684
Total loans					3,215,156			3,362,274
Multifamily real estate [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			1,633	1,360
Provision for loan losses	27	63	2,112	254
Recoveries	0	0	0	11
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	3,745	1,625	3,745	1,625
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0			0
Allowance collectively evaluated for impairment					3,745			1,625
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	3,745	1,625	1,633	1,360	3,745	3,718	1,633	1,625	1,562
Loans individually evaluated for impairment					0			0
Loans collectively evaluated for impairment					309,667			311,533
Purchased Credit-Impaired					142		169	173
Total loans					309,809		314,188	311,706
Construction and Land [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			27,568	34,252
Provision for loan losses	(1,996)	2,037	(2,715)	(6,010)
Recoveries	5	73	190	1,180
Charge-offs	(479)	0	(479)	0
Allowance for loan losses, Ending balance	24,564	29,422	24,564	29,422
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0			67
Allowance collectively evaluated for impairment					24,564			29,348
Allowance for purchased credit-impaired loans					0			7
Total allowance for loan losses	24,564	29,422	27,568	34,252	24,564	27,034	27,568	29,422	27,312
Loans individually evaluated for impairment					1,669			1,871
Loans collectively evaluated for impairment					1,014,140			872,783
Purchased Credit-Impaired					3,236			3,759
Total loans					1,019,045			878,413
Commercial business [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			18,311	16,533
Provision for loan losses	(1,306)	(555)	148	4,489
Recoveries	586	577	856	921
Charge-offs	(473)	(491)	(1,367)	(3,286)
Allowance for loan losses, Ending balance	17,948	18,657	17,948	18,657
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					17			52
Allowance collectively evaluated for impairment					17,908			18,605
Allowance for purchased credit-impaired loans					23			0
Total allowance for loan losses	17,948	18,657	18,311	16,533	17,948	19,141	18,311	18,657	19,126
Loans individually evaluated for impairment					384			5,899
Loans collectively evaluated for impairment					1,357,144			1,302,902
Purchased Credit-Impaired					621		2,159	2,608
Total loans					1,358,149		1,279,894	1,311,409
Agricultural Business/Farmland [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			4,053	2,967
Provision for loan losses	348	1,141	(248)	2,113
Recoveries	0	1	41	133
Charge-offs	(5)	(1,001)	(341)	(1,264)
Allowance for loan losses, Ending balance	3,505	3,949	3,505	3,949
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					71			196
Allowance collectively evaluated for impairment					3,372			3,753
Allowance for purchased credit-impaired loans					62			0
Total allowance for loan losses	3,505	3,949	4,053	2,967	3,505	3,162	4,053	3,949	3,808
Loans individually evaluated for impairment					3,298			6,495
Loans collectively evaluated for impairment					356,270			332,754
Purchased Credit-Impaired					398		565	683
Total loans					359,966		338,388	339,932
One- to four-family [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			2,055	2,238
Provision for loan losses	432	22	1,679	(460)
Recoveries	86	8	732	262
Charge-offs	(27)	0	(43)	0
Allowance for loan losses, Ending balance	4,423	2,040	4,423	2,040
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					64			184
Allowance collectively evaluated for impairment					4,359			1,856
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	4,423	2,040	2,055	2,238	4,423	3,932	2,055	2,040	2,010
Loans individually evaluated for impairment					4,497			5,182
Loans collectively evaluated for impairment					845,320			864,109
Purchased Credit-Impaired					111		136	265
Total loans					849,928		848,289	869,556
Consumer [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			3,866	4,104
Provision for loan losses	2,600	117	4,777	488
Recoveries	46	98	264	293
Charge-offs	(261)	(186)	(797)	(869)
Allowance for loan losses, Ending balance	8,110	4,016	8,110	4,016
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					8			11
Allowance collectively evaluated for impairment					8,102			4,005
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	8,110	4,016	3,866	4,104	8,110	5,725	3,866	4,016	3,987
Loans individually evaluated for impairment					201			218
Loans collectively evaluated for impairment					710,198			700,892
Purchased Credit-Impaired					67			49
Total loans					710,466			701,159
Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			8,718	3,550
Provision for loan losses	1,071	(589)	(897)	2,410
Recoveries	0	0	0	0
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	7,821	5,960	7,821	5,960
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0			0
Allowance collectively evaluated for impairment					7,821			5,960
Allowance for purchased credit-impaired loans					0			0
Total allowance for loan losses	$ 7,821	$ 5,960	$ 8,718	$ 3,550	7,821	$ 6,750	$ 8,718	5,960	$ 6,549
Loans individually evaluated for impairment					0			0
Loans collectively evaluated for impairment					0			0
Purchased Credit-Impaired					0			0
Total loans					$ 0			$ 0