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 CREDIT LOSSES (Allowance for Loan Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2017
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			$ 100,559
Provision for credit losses	$ 13,641	$ 2,000	64,878	$ 6,000
Recoveries	445	710	3,453	1,811
Charge-offs	(2,473)	(3,163)	(8,737)	(6,495)
Allowance for loan losses, Ending balance	167,965	97,801	167,965	97,801
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment									$ 288
Allowance collectively evaluated for impairment									97,439
Allowance for purchased credit-impaired loans									74
Total allowance for loan losses	167,965	97,801	100,559	97,801	$ 167,965	$ 156,352		$ 100,559	97,801	$ 98,254	$ 96,485
Loans individually evaluated for impairment									12,352
Loans collectively evaluated for impairment									8,810,441
Purchased Credit-Impaired								15,938	12,575
Total loans					10,163,917			9,305,357	8,835,368
Commercial real estate
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			30,591
Provision for credit losses	6,895	1,992	32,213	2,244
Recoveries	23	107	244	277
Charge-offs	(379)	(314)	(479)	(1,138)
Allowance for loan losses, Ending balance	59,705	28,515	59,705	28,515
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment									60
Allowance collectively evaluated for impairment									28,455
Allowance for purchased credit-impaired loans									0
Total allowance for loan losses	59,705	28,515	30,591	28,515	59,705	53,166		30,591	28,515	26,730	27,132
Loans individually evaluated for impairment									4,246
Loans collectively evaluated for impairment									3,598,482
Purchased Credit-Impaired									11,513
Total loans									3,614,241
Multifamily real estate
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			4,754
Provision for credit losses	(248)	(61)	772	465
Recoveries	0	0	0	0
Charge-offs	0	0	(66)	0
Allowance for loan losses, Ending balance	3,256	4,283	3,256	4,283
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment									0
Allowance collectively evaluated for impairment									4,283
Allowance for purchased credit-impaired loans									0
Total allowance for loan losses	3,256	4,283	3,256	4,283	3,256	3,504		4,754	4,283	4,344	3,818
Loans individually evaluated for impairment									0
Loans collectively evaluated for impairment									399,808
Purchased Credit-Impaired								7	6
Total loans					426,659			473,152	399,814
Construction and land
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			22,994
Provision for credit losses	2,561	(1,141)	13,963	(2,081)
Recoveries	0	156	105	208
Charge-offs	0	0	(100)	0
Allowance for loan losses, Ending balance	39,477	22,569	39,477	22,569
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment									0
Allowance collectively evaluated for impairment									22,569
Allowance for purchased credit-impaired loans									0
Total allowance for loan losses	39,477	22,569	22,994	22,569	39,477	36,916		22,994	22,569	23,554	24,442
Loans individually evaluated for impairment									1,220
Loans collectively evaluated for impairment									1,083,083
Purchased Credit-Impaired									0
Total loans									1,084,303
Commercial business
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			23,370
Provision for credit losses	2,550	3,027	14,402	4,300
Recoveries	246	162	821	400
Charge-offs	(1,297)	(1,599)	(6,234)	(2,991)
Allowance for loan losses, Ending balance	35,369	21,147	35,369	21,147
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment									48
Allowance collectively evaluated for impairment									21,078
Allowance for purchased credit-impaired loans									21
Total allowance for loan losses	35,369	21,147	23,370	21,147	35,369	33,870		23,370	21,147	19,557	19,438
Loans individually evaluated for impairment									618
Loans collectively evaluated for impairment									1,618,366
Purchased Credit-Impaired								368	407
Total loans					2,343,619			1,693,824	1,619,391
Agricultural business
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			4,120
Provision for credit losses	1,026	943	64	987
Recoveries	0	2	1,772	37
Charge-offs	(492)	(741)	(554)	(907)
Allowance for loan losses, Ending balance	5,051	3,895	5,051	3,895
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment									132
Allowance collectively evaluated for impairment									3,710
Allowance for purchased credit-impaired loans									53
Total allowance for loan losses	5,051	3,895	4,120	3,895	5,051	4,517		4,120	3,895	3,691	3,778
Loans individually evaluated for impairment									2,282
Loans collectively evaluated for impairment									387,827
Purchased Credit-Impaired								393	396
Total loans					326,169			370,549	390,505
One-to four-family residential
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			4,136
Provision for credit losses	100	(175)	1,470	(461)
Recoveries	94	129	273	402
Charge-offs	(72)	(86)	(136)	(86)
Allowance for loan losses, Ending balance	12,868	4,569	12,868	4,569
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment									42
Allowance collectively evaluated for impairment									4,527
Allowance for purchased credit-impaired loans									0
Total allowance for loan losses	12,868	4,569	4,136	4,569	12,868	12,746		4,136	4,569	4,701	4,714
Loans individually evaluated for impairment									3,745
Loans collectively evaluated for impairment									943,653
Purchased Credit-Impaired								74	77
Total loans					771,431			945,622	947,475
Consumer [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			8,202
Provision for credit losses	757	258	1,994	1,355
Recoveries	82	154	238	487
Charge-offs	(233)	(423)	(1,168)	(1,373)
Allowance for loan losses, Ending balance	12,239	8,441	12,239	8,441
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment									6
Allowance collectively evaluated for impairment									8,435
Allowance for purchased credit-impaired loans									0
Total allowance for loan losses	12,239	8,441	8,202	8,441	12,239	11,633		8,202	8,441	8,452	7,972
Loans individually evaluated for impairment									241
Loans collectively evaluated for impairment									779,222
Purchased Credit-Impaired									176
Total loans									779,639
Unallocated
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance			2,392
Provision for credit losses	0	(2,843)	0	(809)
Recoveries	0	0	0	0
Charge-offs	0	0	0	0
Allowance for loan losses, Ending balance	0	4,382	0	4,382
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment									0
Allowance collectively evaluated for impairment									4,382
Allowance for purchased credit-impaired loans									0
Total allowance for loan losses	$ 0	$ 4,382	$ 2,392	$ 4,382	$ 0	$ 0		$ 2,392	4,382	$ 7,225	$ 5,191
Loans individually evaluated for impairment									0
Loans collectively evaluated for impairment									0
Purchased Credit-Impaired									0
Total loans									$ 0
Impact of Topic 326 Adoption
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses							$ 7,812
Impact of Topic 326 Adoption \| Commercial real estate
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses							(2,864)
Impact of Topic 326 Adoption \| Multifamily real estate
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses							(2,204)
Impact of Topic 326 Adoption \| Construction and land
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses							2,515
Impact of Topic 326 Adoption \| Commercial business
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses							3,010
Impact of Topic 326 Adoption \| Agricultural business
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses							(351)
Impact of Topic 326 Adoption \| One-to four-family residential
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses							7,125
Impact of Topic 326 Adoption \| Consumer [Member]
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses							2,973
Impact of Topic 326 Adoption \| Unallocated
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses							$ (2,392)