LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Past Due	$ 35,562	$ 38,202
Carrying balance of purchased credit-impaired loans	15,938	14,413
Current	9,253,857	8,631,980
Loans receivable	9,305,357	8,684,595
Loans 90 Days or More Past Due and Accruing	2,097	906
Financing Receivable, Nonaccrual	37,501	14,748
Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	4,621	3,527
Carrying balance of purchased credit-impaired loans	8,578	8,531
Current	1,567,451	1,418,039
Loans receivable	1,580,650	1,430,097
Loans 90 Days or More Past Due and Accruing	89	0
Financing Receivable, Nonaccrual	4,069	2,768
Commercial real estate - investment properties [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,143	1,523
Carrying balance of purchased credit-impaired loans	6,345	3,462
Current	2,300,733	2,126,074
Loans receivable	2,309,221	2,131,059
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Nonaccrual	1,883	1,320
Multifamily real estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due	330	317
Carrying balance of purchased credit-impaired loans	7	138
Current	472,815	368,381
Loans receivable	473,152	368,836
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Nonaccrual	85	0
Commercial Construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,495	1,427
Carrying balance of purchased credit-impaired loans	0	0
Current	209,173	170,983
Loans receivable	210,668	172,410
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Nonaccrual	98	1,427
Multifamily construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Carrying balance of purchased credit-impaired loans	0	0
Current	233,610	184,630
Loans receivable	233,610	184,630
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Nonaccrual	0	0
One-to four-family construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	5,011	6,778
Carrying balance of purchased credit-impaired loans	0	137
Current	539,297	527,763
Loans receivable	544,308	534,678
Loans 90 Days or More Past Due and Accruing	332	0
Financing Receivable, Nonaccrual	1,467	919
Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	340	1,248
Carrying balance of purchased credit-impaired loans	0	0
Current	154,348	187,260
Loans receivable	154,688	188,508
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Nonaccrual	340	798
Land and land development - commercial [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	78
Carrying balance of purchased credit-impaired loans	0	0
Current	26,290	27,200
Loans receivable	26,290	27,278
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Nonaccrual	0	44
Commercial business [Member]
Financing Receivable, Past Due [Line Items]
Past Due	7,338	7,043
Carrying balance of purchased credit-impaired loans	368	1,028
Current	1,686,118	1,475,543
Loans receivable	1,693,824	1,483,614
Loans 90 Days or More Past Due and Accruing	401	1
Financing Receivable, Nonaccrual	23,015	2,936
Agricultural business, including secured by farmland [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,685	3,462
Carrying balance of purchased credit-impaired loans	393	493
Current	367,471	400,918
Loans receivable	370,549	404,873
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Nonaccrual	661	1,751
One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	7,741	8,887
Carrying balance of purchased credit-impaired loans	74	101
Current	937,807	964,628
Loans receivable	945,622	973,616
Loans 90 Days or More Past Due and Accruing	877	658
Financing Receivable, Nonaccrual	3,410	1,544
Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,347	2,607
Carrying balance of purchased credit-impaired loans	110	432
Current	547,503	565,940
Loans receivable	550,960	568,979
Loans 90 Days or More Past Due and Accruing	398	238
Financing Receivable, Nonaccrual	2,314	1,201
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due	511	1,305
Carrying balance of purchased credit-impaired loans	63	91
Current	211,241	214,621
Loans receivable	211,815	216,017
Loans 90 Days or More Past Due and Accruing	0	9
Financing Receivable, Nonaccrual	159	40
30 to 59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	14,950	18,270
30 to 59 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	486	785
30 to 59 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	91
30 to 59 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due	239	317
30 to 59 Days Past Due [Member] | Commercial Construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,397	0
30 to 59 Days Past Due [Member] | Multifamily construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	0
30 to 59 Days Past Due [Member] | One-to four-family construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,212	4,781
30 to 59 Days Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	450
30 to 59 Days Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	34
30 to 59 Days Past Due [Member] | Commercial business [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,343	3,982
30 to 59 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,972	343
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,777	5,440
30 to 59 Days Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,174	1,136
30 to 59 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due	350	911
60 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	4,885	7,858
60 to 89 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,246	519
60 to 89 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Past Due [Line Items]
Past Due	260	498
60 to 89 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due	91	0
60 to 89 Days Past Due [Member] | Commercial Construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] | Multifamily construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] | One-to four-family construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	1,078
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	0
60 to 89 Days Past Due [Member] | Commercial business [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,583	1,305
60 to 89 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Past Due [Line Items]
Past Due	129	1,518
60 to 89 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,088	1,790
60 to 89 Days Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	327	765
60 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due	161	385
90 Days or More Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	15,727	12,074
90 Days or More Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,889	2,223
90 Days or More Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,883	934
90 Days or More Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	0
90 Days or More Past Due [Member] | Commercial Construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	98	1,427
90 Days or More Past Due [Member] | Multifamily construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	0
90 Days or More Past Due [Member] | One-to four-family construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,799	919
90 Days or More Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	340	798
90 Days or More Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	44
90 Days or More Past Due [Member] | Commercial business [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,412	1,756
90 Days or More Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Past Due [Line Items]
Past Due	584	1,601
90 Days or More Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,876	1,657
90 Days or More Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,846	706
90 Days or More Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Past Due	$ 0	$ 9