LOANS RECEIVABLE AND THE ALLOWANCE FOR CREDIT LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total loans	$ 9,218,384	$ 9,870,982
Non-accrual with no Allowance	15,784	19,586
Nonaccrual loans	24,039	32,560
Loans 90 days or more past due and still accruing	4,822	3,054
Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Total loans	1,122,275	1,076,467
Non-accrual with no Allowance	3,952	7,509
Nonaccrual loans	4,535	8,429
Loans 90 days or more past due and still accruing	0	0
Financing Receivable, before Allowance for Credit Loss	1,122,275	1,076,467
Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Total loans	1,980,284	1,955,684
Non-accrual with no Allowance	7,468	8,979
Nonaccrual loans	7,620	8,979
Loans 90 days or more past due and still accruing	3,955	0
Financing Receivable, before Allowance for Credit Loss	1,980,284	1,955,684
Small balance CRE
Financing Receivable, Past Due [Line Items]
Total loans	601,751	573,849
Non-accrual with no Allowance	2,440	567
Nonaccrual loans	2,776	791
Loans 90 days or more past due and still accruing	0	0
Financing Receivable, before Allowance for Credit Loss	601,751	573,849
Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Total loans	532,760	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	532,760	428,223
Commercial construction
Financing Receivable, Past Due [Line Items]
Total loans	170,205	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	170,205	228,937
Multifamily construction
Financing Receivable, Past Due [Line Items]
Total loans	278,184	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	278,184	305,527
One- to four- family construction
Financing Receivable, Past Due [Line Items]
Total loans	571,431	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	571,431	507,810
Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Total loans	308,164	248,915
Non-accrual with no Allowance	0	302
Nonaccrual loans	256	507
Loans 90 days or more past due and still accruing	0	0
Commercial Business
Financing Receivable, Past Due [Line Items]
Total loans	1,346,707	2,178,461
Non-accrual with no Allowance	820	555
Nonaccrual loans	1,326	1,988
Loans 90 days or more past due and still accruing	0	889
Financing Receivable, before Allowance for Credit Loss	1,346,707	2,178,461
Small business scored
Financing Receivable, Past Due [Line Items]
Total loans	775,554	743,451
Non-accrual with no Allowance	83	91
Nonaccrual loans	1,374	3,419
Loans 90 days or more past due and still accruing	61	136
Financing Receivable, before Allowance for Credit Loss	775,554	743,451
Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Total loans	287,469	299,949
Non-accrual with no Allowance	1,021	1,412
Nonaccrual loans	1,022	1,743
Loans 90 days or more past due and still accruing	0	0
Financing Receivable, before Allowance for Credit Loss	287,469	299,949
One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Total loans	682,368	717,939
Non-accrual with no Allowance	0	171
Nonaccrual loans	3,182	3,556
Loans 90 days or more past due and still accruing	772	1,899
Financing Receivable, before Allowance for Credit Loss	682,368	717,939
Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Total loans	462,819	491,812
Non-accrual with no Allowance	0	0
Nonaccrual loans	1,831	2,697
Loans 90 days or more past due and still accruing	30	130
Consumer-other
Financing Receivable, Past Due [Line Items]
Total loans	98,413	113,958
Non-accrual with no Allowance	0	0
Nonaccrual loans	19	22
Loans 90 days or more past due and still accruing	4	0
30 to 59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	6,772	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	0	0
30 to 59 Days Past Due [Member] | Small balance CRE
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	2	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	4,886	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	117	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	506	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	0	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	107	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	840	459
30 to 59 Days Past Due [Member] | Consumer-other
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	314	236
60 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,187	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	0	0
60 to 89 Days Past Due [Member] | Small balance CRE
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	43	45
60 to 89 Days Past Due [Member] | Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	342	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	0	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	167	31
60 to 89 Days Past Due [Member] | Small business scored
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	328	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	109	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	164	439
60 to 89 Days Past Due [Member] | Consumer-other
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	34	328
90 Days or More Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	10,660	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	495	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	5,185	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	0	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	14	317
90 Days or More Past Due [Member] | Commercial Business
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	847	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	906	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	1,022	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	1,370	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	719	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	18,619	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	495	1,629
Financial Asset, Past Due | Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	5,185	7,981
Financial Asset, Past Due | Small balance CRE
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	45	835
Financial Asset, Past Due | Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	342	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	4,886	0
Financial Asset, Past Due | One- to four- family construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	0	687
Financial Asset, Past Due | Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	14	317
Financial Asset, Past Due | Commercial Business
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,131	5,366
Financial Asset, Past Due | Small business scored
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,740	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	1,022	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	1,586	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	1,723	2,745
Financial Asset, Past Due | Consumer-other
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	352	564
Financial Asset, Not Past Due
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	9,199,765	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	1,121,780	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	1,975,099	1,947,703
Financial Asset, Not Past Due | Small balance CRE
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	601,706	573,014
Financial Asset, Not Past Due | Multifamily Real Estate
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	532,418	428,223
Financial Asset, Not Past Due | Commercial construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	170,107	228,839
Financial Asset, Not Past Due | Multifamily construction
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	273,298	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	571,431	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	308,150	248,598
Financial Asset, Not Past Due | Commercial Business
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,345,576	2,173,095
Financial Asset, Not Past Due | Small business scored
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	773,814	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	286,447	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	680,782	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	461,096	489,067
Financial Asset, Not Past Due | Consumer-other
Financing Receivable, Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss	$ 98,061	$ 113,394