LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Allowance for Loan Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	$ 92,207	$ 86,527	$ 89,028	$ 85,997
Provision for loan losses	2,000	2,000	4,000	4,000
Recoveries	830	1,699	2,928	2,377
Charge-offs	(1,162)	(1,640)	(2,081)	(3,788)
Allowance for loan losses, Ending balance	93,875	88,586	93,875	88,586
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					$ 468		$ 1,080
Allowance collectively evaluated for impairment					93,314		87,493
Allowance for purchased credit-impaired loans					93		13
Total allowance for loan losses	92,207	86,527	89,028	85,997	93,875	$ 89,028	88,586
Loans individually evaluated for impairment					18,409		26,774
Loans collectively evaluated for impairment					7,648,260		7,498,522
Purchased Credit-Impaired					18,063	21,310	26,267
Total loans					7,684,732	7,598,884	7,551,563
Commercial real estate [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	23,461	20,472	22,824	20,993
Provision for loan losses	1,035	3,543	320	2,952
Recoveries	216	264	1,568	334
Charge-offs	(299)	(47)	(299)	(47)
Allowance for loan losses, Ending balance	24,413	24,232	24,413	24,232
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					278		270
Allowance collectively evaluated for impairment					24,135		23,962
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	23,461	20,472	22,824	20,993	24,413	22,824	24,232
Loans individually evaluated for impairment					8,998		8,164
Loans collectively evaluated for impairment					3,155,917		3,306,767
Purchased Credit-Impaired					12,605		18,238
Total loans					3,177,520		3,333,169
Multifamily real estate [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	2,592	1,378	1,633	1,360
Provision for loan losses	1,126	173	2,085	191
Recoveries	0	11	0	11
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	3,718	1,562	3,718	1,562
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0		61
Allowance collectively evaluated for impairment					3,718		1,501
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	2,592	1,378	1,633	1,360	3,718	1,633	1,562
Loans individually evaluated for impairment					0		345
Loans collectively evaluated for impairment					330,220		287,923
Purchased Credit-Impaired					164	169	174
Total loans					330,384	314,188	288,442
Construction and Land [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	28,766	29,464	27,568	34,252
Provision for loan losses	(1,743)	(3,176)	(719)	(8,047)
Recoveries	11	1,024	185	1,107
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	27,034	27,312	27,034	27,312
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0		116
Allowance collectively evaluated for impairment					27,034		27,183
Allowance for purchased credit-impaired loans					0		13
Total allowance for loan losses	28,766	29,464	27,568	34,252	27,034	27,568	27,312
Loans individually evaluated for impairment					750		2,281
Loans collectively evaluated for impairment					976,435		805,411
Purchased Credit-Impaired					3,255		3,810
Total loans					980,440		811,502
Commercial business [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	19,885	19,768	18,311	16,533
Provision for loan losses	(469)	356	1,454	5,044
Recoveries	100	171	270	344
Charge-offs	(375)	(1,169)	(894)	(2,795)
Allowance for loan losses, Ending balance	19,141	19,126	19,141	19,126
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					13		59
Allowance collectively evaluated for impairment					19,095		19,067
Allowance for purchased credit-impaired loans					33		0
Total allowance for loan losses	19,885	19,768	18,311	16,533	19,141	18,311	19,126
Loans individually evaluated for impairment					369		6,737
Loans collectively evaluated for impairment					1,310,601		1,276,499
Purchased Credit-Impaired					1,454	2,159	2,968
Total loans					1,312,424	1,279,894	1,286,204
Agricultural Business/Farmland [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	2,999	3,245	4,053	2,967
Provision for loan losses	451	648	(596)	972
Recoveries	41	19	41	132
Charge-offs	(329)	(104)	(336)	(263)
Allowance for loan losses, Ending balance	3,162	3,808	3,162	3,808
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					59		238
Allowance collectively evaluated for impairment					3,043		3,570
Allowance for purchased credit-impaired loans					60		0
Total allowance for loan losses	2,999	3,245	4,053	2,967	3,162	4,053	3,808
Loans individually evaluated for impairment					3,298		3,799
Loans collectively evaluated for impairment					333,015		339,883
Purchased Credit-Impaired					396	565	730
Total loans					336,709	338,388	344,412
One- to four-family [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	3,779	1,974	2,055	2,238
Provision for loan losses	(203)	(73)	1,247	(482)
Recoveries	356	109	646	254
Charge-offs	0	0	(16)	0
Allowance for loan losses, Ending balance	3,932	2,010	3,932	2,010
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					108		324
Allowance collectively evaluated for impairment					3,824		1,686
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	3,779	1,974	2,055	2,238	3,932	2,055	2,010
Loans individually evaluated for impairment					4,789		5,228
Loans collectively evaluated for impairment					835,560		794,486
Purchased Credit-Impaired					121	136	294
Total loans					840,470	848,289	800,008
Consumer [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	5,514	3,840	3,866	4,104
Provision for loan losses	264	366	2,177	371
Recoveries	106	101	218	195
Charge-offs	(159)	(320)	(536)	(683)
Allowance for loan losses, Ending balance	5,725	3,987	5,725	3,987
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					10		12
Allowance collectively evaluated for impairment					5,715		3,975
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	5,514	3,840	3,866	4,104	5,725	3,866	3,987
Loans individually evaluated for impairment					205		220
Loans collectively evaluated for impairment					706,512		687,553
Purchased Credit-Impaired					68		53
Total loans					706,785		687,826
Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	5,211	6,386	8,718	3,550
Provision for loan losses	1,539	163	(1,968)	2,999
Recoveries	0	0	0	0
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	6,750	6,549	6,750	6,549
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0		0
Allowance collectively evaluated for impairment					6,750		6,549
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	$ 5,211	$ 6,386	$ 8,718	$ 3,550	6,750	$ 8,718	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