Reserve for loans impairments recorded after acquisition as a result of current information and events which result in it being probable that estimated cash flows will be less than previously expected or because actual cash flows are less than cash flows previously expected. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that was acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

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LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Allowance for Loan Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	$ 98,254	$ 93,875	$ 96,485	$ 89,028
Provision for loan losses	2,000	2,000	6,000	6,000
Recoveries	710	735	1,811	3,663
Charge-offs	(3,163)	(1,347)	(6,495)	(3,428)
Allowance for loan losses, Ending balance	97,801	95,263	97,801	95,263
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					$ 288		$ 457
Allowance collectively evaluated for impairment					97,439		94,721
Allowance for purchased credit-impaired loans					74		85
Total allowance for loan losses	97,801	93,875	97,801	89,028	97,801	$ 96,485	95,263
Loans individually evaluated for impairment					12,352		18,818
Loans collectively evaluated for impairment					8,810,441		7,790,757
Purchased Credit-Impaired					12,575	14,413	12,944
Total loans					8,835,368	8,684,595	7,822,519
Commercial real estate [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	26,730	24,413	27,132	22,824
Provision for loan losses	1,992	824	2,244	1,144
Recoveries	107	12	277	1,580
Charge-offs	(314)	(102)	(1,138)	(401)
Allowance for loan losses, Ending balance	28,515	25,147	28,515	25,147
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					60		297
Allowance collectively evaluated for impairment					28,455		24,850
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	26,730	24,413	27,132	25,147	28,515	27,132	25,147
Loans individually evaluated for impairment					4,246		8,769
Loans collectively evaluated for impairment					3,598,482		3,198,018
Purchased Credit-Impaired					11,513		8,369
Total loans					3,614,241		3,215,156
Multifamily real estate [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	4,344	3,718	3,818	1,633
Provision for loan losses	(61)	27	465	2,112
Recoveries	0	0	0	0
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	4,283	3,745	4,283	3,745
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0		0
Allowance collectively evaluated for impairment					4,283		3,745
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	4,344	3,718	3,818	1,633	4,283	3,818	3,745
Loans individually evaluated for impairment					0		0
Loans collectively evaluated for impairment					399,808		309,667
Purchased Credit-Impaired					6	138	142
Total loans					399,814	368,836	309,809
Construction and Land [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	23,554	27,034	24,442	27,568
Provision for loan losses	(1,141)	(1,996)	(2,081)	(2,715)
Recoveries	156	5	208	190
Charge-offs	0	(479)	0	(479)
Allowance for loan losses, Ending balance	22,569	24,564	22,569	24,564
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0		0
Allowance collectively evaluated for impairment					22,569		24,564
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	23,554	27,034	22,569	27,568	22,569	24,442	24,564
Loans individually evaluated for impairment					1,220		1,669
Loans collectively evaluated for impairment					1,083,083		1,014,140
Purchased Credit-Impaired					0		3,236
Total loans					1,084,303		1,019,045
Commercial business [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	19,557	19,141	19,438	18,311
Provision for loan losses	3,027	(1,306)	4,300	148
Recoveries	162	586	400	856
Charge-offs	(1,599)	(473)	(2,991)	(1,367)
Allowance for loan losses, Ending balance	21,147	17,948	21,147	17,948
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					48		17
Allowance collectively evaluated for impairment					21,078		17,908
Allowance for purchased credit-impaired loans					21		23
Total allowance for loan losses	19,557	19,141	21,147	17,948	21,147	19,438	17,948
Loans individually evaluated for impairment					618		384
Loans collectively evaluated for impairment					1,618,366		1,357,144
Purchased Credit-Impaired					407	1,028	621
Total loans					1,619,391	1,483,614	1,358,149
Agricultural Business/Farmland [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	3,691	3,162	3,778	4,053
Provision for loan losses	943	348	987	(248)
Recoveries	2	0	37	41
Charge-offs	(741)	(5)	(907)	(341)
Allowance for loan losses, Ending balance	3,895	3,505	3,895	3,505
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					132		71
Allowance collectively evaluated for impairment					3,710		3,372
Allowance for purchased credit-impaired loans					53		62
Total allowance for loan losses	3,691	3,162	3,778	4,053	3,895	3,778	3,505
Loans individually evaluated for impairment					2,282		3,298
Loans collectively evaluated for impairment					387,827		356,270
Purchased Credit-Impaired					396	493	398
Total loans					390,505	404,873	359,966
One- to four-family [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	4,701	3,932	4,714	2,055
Provision for loan losses	(175)	432	(461)	1,679
Recoveries	129	86	402	732
Charge-offs	(86)	(27)	(86)	(43)
Allowance for loan losses, Ending balance	4,569	4,423	4,569	4,423
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					42		64
Allowance collectively evaluated for impairment					4,527		4,359
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	4,701	3,932	4,714	2,055	4,569	4,714	4,423
Loans individually evaluated for impairment					3,745		4,497
Loans collectively evaluated for impairment					943,653		845,320
Purchased Credit-Impaired					77	101	111
Total loans					947,475	973,616	849,928
Consumer [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	8,452	5,725	7,972	3,866
Provision for loan losses	258	2,600	1,355	4,777
Recoveries	154	46	487	264
Charge-offs	(423)	(261)	(1,373)	(797)
Allowance for loan losses, Ending balance	8,441	8,110	8,441	8,110
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					6		8
Allowance collectively evaluated for impairment					8,435		8,102
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	8,452	5,725	7,972	3,866	8,441	7,972	8,110
Loans individually evaluated for impairment					241		201
Loans collectively evaluated for impairment					779,222		710,198
Purchased Credit-Impaired					176		67
Total loans					779,639		710,466
Unallocated Financing Receivables [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	7,225	6,750	5,191	8,718
Provision for loan losses	(2,843)	1,071	(809)	(897)
Recoveries	0	0	0	0
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	4,382	7,821	4,382	7,821
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment					0		0
Allowance collectively evaluated for impairment					4,382		7,821
Allowance for purchased credit-impaired loans					0		0
Total allowance for loan losses	$ 7,225	$ 6,750	$ 5,191	$ 8,718	4,382	$ 5,191	7,821
Loans individually evaluated for impairment					0		0
Loans collectively evaluated for impairment					0		0
Purchased Credit-Impaired					0		0
Total loans					$ 0		$ 0