LOANS RECEIVABLE AND THE ALLOWANCE FOR CREDIT LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Financing Receivable, Past Due [Line Items]
Past Due	$ 37,464	$ 35,562
Purchased Credit-Impaired		15,938	$ 12,575
Current	10,126,453	9,253,857
Total loans	10,163,917	9,305,357	8,835,368
Non-accrual with no Allowance	15,687
Loans 90 days or more past due and still accruing	31,568	2,097
Nonaccrual loans	3,255	37,501
Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,787	4,621
Purchased Credit-Impaired		8,578
Current	1,048,090	1,567,451
Total loans	1,049,877	1,580,650
Non-accrual with no Allowance	1,101
Loans 90 days or more past due and still accruing	2,067	89
Nonaccrual loans	0	4,069
Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Past Due	8,919	2,143
Purchased Credit-Impaired		6,345
Current	1,982,339	2,300,733
Total loans	1,991,258	2,309,221
Non-accrual with no Allowance	3,914
Loans 90 days or more past due and still accruing	3,914	0
Nonaccrual loans	0	1,883
Small balance CRE
Financing Receivable, Past Due [Line Items]
Past Due	2,432
Current	595,539
Total loans	597,971
Non-accrual with no Allowance	1,206
Loans 90 days or more past due and still accruing	1,843
Nonaccrual loans	0
Multifamily real estate
Financing Receivable, Past Due [Line Items]
Past Due	0	330
Purchased Credit-Impaired		7	6
Current	426,659	472,815
Total loans	426,659	473,152	399,814
Non-accrual with no Allowance	0
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	0	85
Commercial construction
Financing Receivable, Past Due [Line Items]
Past Due	99	1,495
Purchased Credit-Impaired		0
Current	220,186	209,173
Total loans	220,285	210,668
Non-accrual with no Allowance	0
Loans 90 days or more past due and still accruing	98	0
Nonaccrual loans	0	98
Multifamily construction
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Purchased Credit-Impaired		0
Current	291,105	233,610
Total loans	291,105	233,610
Non-accrual with no Allowance	0
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	0	0
One- to four- family construction
Financing Receivable, Past Due [Line Items]
Past Due	863	5,011
Purchased Credit-Impaired		0
Current	517,222	539,297
Total loans	518,085	544,308
Non-accrual with no Allowance	0
Loans 90 days or more past due and still accruing	331	332
Nonaccrual loans	0	1,467
Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	508	340
Purchased Credit-Impaired		0
Current	240,295	154,348
Total loans	240,803	154,688
Non-accrual with no Allowance	302
Loans 90 days or more past due and still accruing	508	0
Nonaccrual loans	0	340
Land and land development - commercial [Member]
Financing Receivable, Past Due [Line Items]
Past Due		0
Purchased Credit-Impaired		0
Current		26,290
Total loans		26,290
Loans 90 days or more past due and still accruing		0
Nonaccrual loans		0
Commercial business
Financing Receivable, Past Due [Line Items]
Past Due	5,786	7,338
Purchased Credit-Impaired		368	407
Current	2,337,833	1,686,118
Total loans	2,343,619	1,693,824	1,619,391
Non-accrual with no Allowance	7,468
Loans 90 days or more past due and still accruing	12,143	401
Nonaccrual loans	225	23,015
Small business scored
Financing Receivable, Past Due [Line Items]
Past Due	5,813
Current	758,011
Total loans	763,824
Non-accrual with no Allowance	93
Loans 90 days or more past due and still accruing	2,724
Nonaccrual loans	200
Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Past Due	2,023	2,685
Purchased Credit-Impaired		393	396
Current	324,146	367,471
Total loans	326,169	370,549	390,505
Non-accrual with no Allowance	1,422
Loans 90 days or more past due and still accruing	2,066	0
Nonaccrual loans	0	661
One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	5,611	7,741
Purchased Credit-Impaired		74	77
Current	765,820	937,807
Total loans	771,431	945,622	$ 947,475
Non-accrual with no Allowance	181
Loans 90 days or more past due and still accruing	2,978	877
Nonaccrual loans	2,649	3,410
Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,158	3,347
Purchased Credit-Impaired		110
Current	501,365	547,503
Total loans	504,523	550,960
Non-accrual with no Allowance	0
Loans 90 days or more past due and still accruing	2,835	398
Nonaccrual loans	175	2,314
Consumer
Financing Receivable, Past Due [Line Items]
Past Due	465	511
Purchased Credit-Impaired		63
Current	117,843	211,241
Total loans	118,308	211,815
Non-accrual with no Allowance	0
Loans 90 days or more past due and still accruing	61	0
Nonaccrual loans	6	159
Consumer Loan in Effect Before Topic 326 [Member]
Financing Receivable, Past Due [Line Items]
Total loans		211,815
30 to 59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	11,315	14,950
30 to 59 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	100	486
30 to 59 Days Past Due [Member] | Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Past Due	5,902	0
30 to 59 Days Past Due [Member] | Small balance CRE
Financing Receivable, Past Due [Line Items]
Past Due	610
30 to 59 Days Past Due [Member] | Multifamily real estate
Financing Receivable, Past Due [Line Items]
Past Due	0	239
30 to 59 Days Past Due [Member] | Commercial construction
Financing Receivable, Past Due [Line Items]
Past Due	0	1,397
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	441	3,212
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] | Land and land development - commercial [Member]
Financing Receivable, Past Due [Line Items]
Past Due		0
30 to 59 Days Past Due [Member] | Commercial business
Financing Receivable, Past Due [Line Items]
Past Due	485	2,343
30 to 59 Days Past Due [Member] | Small business scored
Financing Receivable, Past Due [Line Items]
Past Due	2,491
30 to 59 Days Past Due [Member] | Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Past Due	0	1,972
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	230	3,777
30 to 59 Days Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	873	1,174
30 to 59 Days Past Due [Member] | Consumer
Financing Receivable, Past Due [Line Items]
Past Due	183	350
60 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	8,297	4,885
60 to 89 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	1,246
60 to 89 Days Past Due [Member] | Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Past Due	98	260
60 to 89 Days Past Due [Member] | Small balance CRE
Financing Receivable, Past Due [Line Items]
Past Due	622
60 to 89 Days Past Due [Member] | Multifamily real estate
Financing Receivable, Past Due [Line Items]
Past Due	0	91
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	422	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] | Land and land development - commercial [Member]
Financing Receivable, Past Due [Line Items]
Past Due		0
60 to 89 Days Past Due [Member] | Commercial business
Financing Receivable, Past Due [Line Items]
Past Due	3,640	1,583
60 to 89 Days Past Due [Member] | Small business scored
Financing Receivable, Past Due [Line Items]
Past Due	1,298
60 to 89 Days Past Due [Member] | Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Past Due	0	129
60 to 89 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,749	1,088
60 to 89 Days Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	215	327
60 to 89 Days Past Due [Member] | Consumer
Financing Receivable, Past Due [Line Items]
Past Due	253	161
90 Days or More Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	17,852	15,727
90 Days or More Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,687	2,889
90 Days or More Past Due [Member] | Commercial real estate - investment properties
Financing Receivable, Past Due [Line Items]
Past Due	2,919	1,883
90 Days or More Past Due [Member] | Small balance CRE
Financing Receivable, Past Due [Line Items]
Past Due	1,200
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	99	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	1,799
90 Days or More Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	508	340
90 Days or More Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Past Due [Line Items]
Past Due		0
90 Days or More Past Due [Member] | Commercial business
Financing Receivable, Past Due [Line Items]
Past Due	1,661	3,412
90 Days or More Past Due [Member] | Small business scored
Financing Receivable, Past Due [Line Items]
Past Due	2,024
90 Days or More Past Due [Member] | Agricultural business, including secured by farmland
Financing Receivable, Past Due [Line Items]
Past Due	2,023	584
90 Days or More Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,632	2,876
90 Days or More Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,070	1,846
90 Days or More Past Due [Member] | Consumer
Financing Receivable, Past Due [Line Items]
Past Due	$ 29	$ 0