LOANS RECEIVABLE AND THE ALLOWANCE FOR CREDIT LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total loans	$ 9,654,181	$ 9,870,982
Non-accrual with no Allowance	18,167	19,586
Nonaccrual loans	28,647	32,560
Loans 90 days or more past due and still accruing	2,116	3,054
Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Total loans	1,066,237	1,076,467
Non-accrual with no Allowance	4,687	7,509
Nonaccrual loans	5,322	8,429
Loans 90 days or more past due and still accruing	911	0
Financing Receivable, before Allowance for Credit Loss	1,066,237	1,076,467
Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Total loans	1,950,211	1,955,684
Non-accrual with no Allowance	8,708	8,979
Nonaccrual loans	8,708	8,979
Loans 90 days or more past due and still accruing	0	0
Financing Receivable, before Allowance for Credit Loss	1,950,211	1,955,684
Small balance CRE
Financing Receivable, Past Due [Line Items]
Total loans	621,102	573,849
Non-accrual with no Allowance	3,045	567
Nonaccrual loans	3,397	791
Loans 90 days or more past due and still accruing	0	0
Financing Receivable, before Allowance for Credit Loss	621,102	573,849
Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Total loans	504,445	428,223
Non-accrual with no Allowance	0	0
Nonaccrual loans	0	0
Loans 90 days or more past due and still accruing	0	0
Financing Receivable, before Allowance for Credit Loss	504,445	428,223
Commercial construction
Financing Receivable, Past Due [Line Items]
Total loans	182,868	228,937
Non-accrual with no Allowance	0	0
Nonaccrual loans	98	98
Loans 90 days or more past due and still accruing	0	0
Financing Receivable, before Allowance for Credit Loss	182,868	228,937
Multifamily construction
Financing Receivable, Past Due [Line Items]
Total loans	295,661	305,527
Non-accrual with no Allowance	0	0
Nonaccrual loans	0	0
Loans 90 days or more past due and still accruing	0	0
Financing Receivable, before Allowance for Credit Loss	295,661	305,527
One- to four- family construction
Financing Receivable, Past Due [Line Items]
Total loans	603,895	507,810
Non-accrual with no Allowance	0	0
Nonaccrual loans	0	331
Loans 90 days or more past due and still accruing	0	0
Financing Receivable, before Allowance for Credit Loss	603,895	507,810
Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Total loans	290,404	248,915
Non-accrual with no Allowance	0	302
Nonaccrual loans	443	507
Loans 90 days or more past due and still accruing	0	0
Commercial Business
Financing Receivable, Past Due [Line Items]
Total loans	1,931,531	2,178,461
Non-accrual with no Allowance	619	555
Nonaccrual loans	1,228	1,988
Loans 90 days or more past due and still accruing	33	889
Financing Receivable, before Allowance for Credit Loss	1,931,531	2,178,461
Small business scored
Financing Receivable, Past Due [Line Items]
Total loans	743,975	743,451
Non-accrual with no Allowance	87	91
Nonaccrual loans	2,445	3,419
Loans 90 days or more past due and still accruing	462	136
Financing Receivable, before Allowance for Credit Loss	743,975	743,451
Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Total loans	265,429	299,949
Non-accrual with no Allowance	1,021	1,412
Nonaccrual loans	1,200	1,743
Loans 90 days or more past due and still accruing	0	0
Financing Receivable, before Allowance for Credit Loss	265,429	299,949
One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Total loans	637,701	717,939
Non-accrual with no Allowance	0	171
Nonaccrual loans	4,007	3,556
Loans 90 days or more past due and still accruing	579	1,899
Financing Receivable, before Allowance for Credit Loss	637,701	717,939
Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Total loans	458,915	491,812
Non-accrual with no Allowance	0	0
Nonaccrual loans	1,778	2,697
Loans 90 days or more past due and still accruing	131	130
Consumer-other
Financing Receivable, Past Due [Line Items]
Total loans	101,807	113,958
Non-accrual with no Allowance	0	0
Nonaccrual loans	21	22
Loans 90 days or more past due and still accruing	0	0
30 to 59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	5,088	10,675
30 to 59 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
30 to 59 Days Past Due [Member] | Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	649	0
30 to 59 Days Past Due [Member] | Small balance CRE
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	676	0
30 to 59 Days Past Due [Member] | Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
30 to 59 Days Past Due [Member] | Commercial construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
30 to 59 Days Past Due [Member] | Multifamily construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
30 to 59 Days Past Due [Member] | One- to four- family construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	356
30 to 59 Days Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
30 to 59 Days Past Due [Member] | Commercial Business
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,708	3,247
30 to 59 Days Past Due [Member] | Small business scored
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,360	1,459
30 to 59 Days Past Due [Member] | Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	200	298
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	87	4,620
30 to 59 Days Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	231	459
30 to 59 Days Past Due [Member] | Consumer-other
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	177	236
60 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	2,526	2,908
60 to 89 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	182
60 to 89 Days Past Due [Member] | Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,276	0
60 to 89 Days Past Due [Member] | Small balance CRE
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	45
60 to 89 Days Past Due [Member] | Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
60 to 89 Days Past Due [Member] | Commercial construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
60 to 89 Days Past Due [Member] | Multifamily construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
60 to 89 Days Past Due [Member] | One- to four- family construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	175	0
60 to 89 Days Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
60 to 89 Days Past Due [Member] | Commercial Business
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	284	31
60 to 89 Days Past Due [Member] | Small business scored
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	192	891
60 to 89 Days Past Due [Member] | Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	37
60 to 89 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	540	955
60 to 89 Days Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	24	439
60 to 89 Days Past Due [Member] | Consumer-other
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	35	328
90 Days or More Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	15,968	22,548
90 Days or More Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	6,141	1,447
90 Days or More Past Due [Member] | Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	7,981
90 Days or More Past Due [Member] | Small balance CRE
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,978	790
90 Days or More Past Due [Member] | Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
90 Days or More Past Due [Member] | Commercial construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	98	98
90 Days or More Past Due [Member] | Multifamily construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
90 Days or More Past Due [Member] | One- to four- family construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	331
90 Days or More Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	260	317
90 Days or More Past Due [Member] | Commercial Business
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	818	2,088
90 Days or More Past Due [Member] | Small business scored
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,901	2,048
90 Days or More Past Due [Member] | Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,200	1,548
90 Days or More Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	2,402	4,053
90 Days or More Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,166	1,847
90 Days or More Past Due [Member] | Consumer-other
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	4	0
Financial Asset, Past Due
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	23,582	36,131
Financial Asset, Past Due | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	6,141	1,629
Financial Asset, Past Due | Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,925	7,981
Financial Asset, Past Due | Small balance CRE
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	2,654	835
Financial Asset, Past Due | Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
Financial Asset, Past Due | Commercial construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	98	98
Financial Asset, Past Due | Multifamily construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	0
Financial Asset, Past Due | One- to four- family construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	175	687
Financial Asset, Past Due | Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	260	317
Financial Asset, Past Due | Commercial Business
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	2,810	5,366
Financial Asset, Past Due | Small business scored
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	3,453	4,398
Financial Asset, Past Due | Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,400	1,883
Financial Asset, Past Due | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	3,029	9,628
Financial Asset, Past Due | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,421	2,745
Financial Asset, Past Due | Consumer-other
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	216	564
Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	9,630,599	9,834,851
Financial Asset, Not Past Due | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,060,096	1,074,838
Financial Asset, Not Past Due | Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,948,286	1,947,703
Financial Asset, Not Past Due | Small balance CRE
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	618,448	573,014
Financial Asset, Not Past Due | Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	504,445	428,223
Financial Asset, Not Past Due | Commercial construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	182,770	228,839
Financial Asset, Not Past Due | Multifamily construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	295,661	305,527
Financial Asset, Not Past Due | One- to four- family construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	603,720	507,123
Financial Asset, Not Past Due | Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	290,144	248,598
Financial Asset, Not Past Due | Commercial Business
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,928,721	2,173,095
Financial Asset, Not Past Due | Small business scored
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	740,522	739,053
Financial Asset, Not Past Due | Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	264,029	298,066
Financial Asset, Not Past Due | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	634,672	708,311
Financial Asset, Not Past Due | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	457,494	489,067
Financial Asset, Not Past Due | Consumer-other
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	$ 101,591	$ 113,394