LOANS RECEIVABLE AND THE ALLOWANCE FOR CREDIT LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Past Due	$ 42,444	$ 36,131
Current	9,905,253	9,834,851
Total loans	9,947,697	9,870,982
Non-accrual with no Allowance	22,416	19,586
Nonaccrual loans	35,031	32,560
Loans 90 days or more past due and still accruing	1,561	3,054
Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	20,098	1,629
Current	1,025,558	1,074,838
Total loans	1,045,656	1,076,467
Non-accrual with no Allowance	4,687	7,509
Nonaccrual loans	5,416	8,429
Loans 90 days or more past due and still accruing	0	0
Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Past Due	5,032	7,981
Current	1,926,773	1,947,703
Total loans	1,931,805	1,955,684
Non-accrual with no Allowance	12,568	8,979
Nonaccrual loans	12,568	8,979
Loans 90 days or more past due and still accruing	0	0
Small balance CRE
Financing Receivable, Past Due [Line Items]
Past Due	3,654	835
Current	635,676	573,014
Total loans	639,330	573,849
Non-accrual with no Allowance	3,086	567
Nonaccrual loans	3,631	791
Loans 90 days or more past due and still accruing	0	0
Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Current	433,775	428,223
Total loans	433,775	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
Commercial construction
Financing Receivable, Past Due [Line Items]
Past Due	98	98
Current	198,939	228,839
Total loans	199,037	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
Multifamily construction
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Current	305,694	305,527
Total loans	305,694	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
One- to four- family construction
Financing Receivable, Past Due [Line Items]
Past Due	170	687
Current	542,670	507,123
Total loans	542,840	507,810
Non-accrual with no Allowance	0	0
Nonaccrual loans	687	331
Loans 90 days or more past due and still accruing	0	0
Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	14	317
Current	266,716	248,598
Total loans	266,730	248,915
Non-accrual with no Allowance	0	302
Nonaccrual loans	201	507
Loans 90 days or more past due and still accruing	0	0
Commercial Business
Financing Receivable, Past Due [Line Items]
Past Due	945	5,366
Current	2,375,649	2,173,095
Total loans	2,376,594	2,178,461
Non-accrual with no Allowance	628	555
Nonaccrual loans	1,266	1,988
Loans 90 days or more past due and still accruing	0	889
Small business scored
Financing Receivable, Past Due [Line Items]
Past Due	4,050	4,398
Current	713,452	739,053
Total loans	717,502	743,451
Non-accrual with no Allowance	90	91
Nonaccrual loans	2,928	3,419
Loans 90 days or more past due and still accruing	37	136
Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Past Due	1,536	1,883
Current	260,874	298,066
Total loans	262,410	299,949
Non-accrual with no Allowance	1,357	1,412
Nonaccrual loans	1,536	1,743
Loans 90 days or more past due and still accruing	0	0
One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	4,561	9,628
Current	651,066	708,311
Total loans	655,627	717,939
Non-accrual with no Allowance	0	171
Nonaccrual loans	4,456	3,556
Loans 90 days or more past due and still accruing	1,524	1,899
Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,097	2,745
Current	464,035	489,067
Total loans	466,132	491,812
Non-accrual with no Allowance	0	0
Nonaccrual loans	2,234	2,697
Loans 90 days or more past due and still accruing	0	130
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due	189	564
Current	104,376	113,394
Total loans	104,565	113,958
Non-accrual with no Allowance	0	0
Nonaccrual loans	10	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]
Past Due	19,091	10,675
30 to 59 Days Past Due [Member] \| Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	14,824	0
30 to 59 Days Past Due [Member] \| Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Past Due	1	0
30 to 59 Days Past Due [Member] \| Small balance CRE
Financing Receivable, Past Due [Line Items]
Past Due	895	0
30 to 59 Days Past Due [Member] \| Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Past Due	0	0
30 to 59 Days Past Due [Member] \| Commercial construction
Financing Receivable, Past Due [Line Items]
Past Due	0	0
30 to 59 Days Past Due [Member] \| Multifamily construction
Financing Receivable, Past Due [Line Items]
Past Due	0	0
30 to 59 Days Past Due [Member] \| One- to four- family construction
Financing Receivable, Past Due [Line Items]
Past Due	170	356
30 to 59 Days Past Due [Member] \| Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	0
30 to 59 Days Past Due [Member] \| Commercial Business
Financing Receivable, Past Due [Line Items]
Past Due	159	3,247
30 to 59 Days Past Due [Member] \| Small business scored
Financing Receivable, Past Due [Line Items]
Past Due	1,460	1,459
30 to 59 Days Past Due [Member] \| Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Past Due	0	298
30 to 59 Days Past Due [Member] \| One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	987	4,620
30 to 59 Days Past Due [Member] \| Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	464	459
30 to 59 Days Past Due [Member] \| Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due	131	236
60 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	9,681	2,908
60 to 89 Days Past Due [Member] \| Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	4,687	182
60 to 89 Days Past Due [Member] \| Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Past Due	1,322	0
60 to 89 Days Past Due [Member] \| Small balance CRE
Financing Receivable, Past Due [Line Items]
Past Due	1,978	45
60 to 89 Days Past Due [Member] \| Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] \| Commercial construction
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] \| Multifamily construction
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] \| One- to four- family construction
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] \| Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] \| Commercial Business
Financing Receivable, Past Due [Line Items]
Past Due	558	31
60 to 89 Days Past Due [Member] \| Small business scored
Financing Receivable, Past Due [Line Items]
Past Due	948	891
60 to 89 Days Past Due [Member] \| Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Past Due	0	37
60 to 89 Days Past Due [Member] \| One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	21	955
60 to 89 Days Past Due [Member] \| Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	109	439
60 to 89 Days Past Due [Member] \| Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due	58	328
90 Days or More Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	13,672	22,548
90 Days or More Past Due [Member] \| Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	587	1,447
90 Days or More Past Due [Member] \| Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Past Due	3,709	7,981
90 Days or More Past Due [Member] \| Small balance CRE
Financing Receivable, Past Due [Line Items]
Past Due	781	790
90 Days or More Past Due [Member] \| Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Past Due	0	0
90 Days or More Past Due [Member] \| Commercial construction
Financing Receivable, Past Due [Line Items]
Past Due	98	98
90 Days or More Past Due [Member] \| Multifamily construction
Financing Receivable, Past Due [Line Items]
Past Due	0	0
90 Days or More Past Due [Member] \| One- to four- family construction
Financing Receivable, Past Due [Line Items]
Past Due	0	331
90 Days or More Past Due [Member] \| Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	14	317
90 Days or More Past Due [Member] \| Commercial Business
Financing Receivable, Past Due [Line Items]
Past Due	228	2,088
90 Days or More Past Due [Member] \| Small business scored
Financing Receivable, Past Due [Line Items]
Past Due	1,642	2,048
90 Days or More Past Due [Member] \| Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Past Due	1,536	1,548
90 Days or More Past Due [Member] \| One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,553	4,053
90 Days or More Past Due [Member] \| Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,524	1,847
90 Days or More Past Due [Member] \| Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due	$ 0	$ 0

LOANS RECEIVABLE AND THE ALLOWANCE FOR CREDIT LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($)
$ in Thousands

Mar. 31, 2021

Dec. 31, 2020

Financing Receivable, Past Due [Line Items]

Past Due

$ 42,444

$ 36,131

Current

9,905,253

9,834,851

Total loans

9,947,697