LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Financing Receivable, Past Due [Line Items]
Past Due	$ 24,419	$ 38,202
Purchased Credit-Impaired	12,575	14,413	$ 12,944
Current	8,798,374	8,631,980
Total loans	8,835,368	8,684,595	7,822,519
Loans 90 days or more past due and still accruing	2,487	906
Nonaccrual loans	15,769	14,748
Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,746	3,527
Purchased Credit-Impaired	8,241	8,531
Current	1,451,316	1,418,039
Total loans	1,463,303	1,430,097
Loans 90 days or more past due and still accruing	89	0
Nonaccrual loans	3,040	2,768
Commercial real estate - investment properties [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,300	1,523
Purchased Credit-Impaired	3,272	3,462
Current	2,145,366	2,126,074
Total loans	2,150,938	2,131,059
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	2,052	1,320
Multifamily real estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	317
Purchased Credit-Impaired	6	138	142
Current	399,808	368,381
Total loans	399,814	368,836	309,809
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	87	0
Commercial construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	98	1,427
Purchased Credit-Impaired	0	0
Current	190,434	170,983
Total loans	190,532	172,410
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	98	1,427
Multifamily Construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	0
Purchased Credit-Impaired	0	0
Current	214,878	184,630
Total loans	214,878	184,630
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	0	0
One- to four-family construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,074	6,778
Purchased Credit-Impaired	0	137
Current	485,871	527,763
Total loans	488,945	534,678
Loans 90 days or more past due and still accruing	1,141	0
Nonaccrual loans	578	919
Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	642	1,248
Purchased Credit-Impaired	0	0
Current	163,187	187,260
Total loans	163,829	188,508
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	642	798
Land and land development - commercial [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	78
Purchased Credit-Impaired	0	0
Current	26,119	27,200
Total loans	26,119	27,278
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	0	44
Commercial business [Member]
Financing Receivable, Past Due [Line Items]
Past Due	5,582	7,043
Purchased Credit-Impaired	407	1,028	621
Current	1,613,402	1,475,543
Total loans	1,619,391	1,483,614	1,358,149
Loans 90 days or more past due and still accruing	358	1
Nonaccrual loans	3,035	2,936
Agricultural business, including secured by farmland [Member]
Financing Receivable, Past Due [Line Items]
Past Due	688	3,462
Purchased Credit-Impaired	396	493	398
Current	389,421	400,918
Total loans	390,505	404,873	359,966
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	757	1,751
One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,979	8,887
Purchased Credit-Impaired	77	101	111
Current	943,419	964,628
Total loans	947,475	973,616	$ 849,928
Loans 90 days or more past due and still accruing	652	658
Nonaccrual loans	3,007	1,544
Consumer secured by one- to four-family [Member]
Financing Receivable, Past Due [Line Items]
Past Due	3,745	2,607
Purchased Credit-Impaired	112	432
Current	562,935	565,940
Total loans	566,792	568,979
Loans 90 days or more past due and still accruing	231	238
Nonaccrual loans	2,140	1,201
Consumer - other [Member]
Financing Receivable, Past Due [Line Items]
Past Due	565	1,305
Purchased Credit-Impaired	64	91
Current	212,218	214,621
Total loans	212,847	216,017
Loans 90 days or more past due and still accruing	16	9
Nonaccrual loans	333	40
30 to 59 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	6,366	18,270
30 to 59 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	1,102	785
30 to 59 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Past Due [Line Items]
Past Due	248	91
30 to 59 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	317
30 to 59 Days Past Due [Member] | Commercial construction [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	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	0	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	2,301	3,982
30 to 59 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	343
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	852	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	1,433	1,136
30 to 59 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Past Due [Line Items]
Past Due	430	911
60 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	4,360	7,858
60 to 89 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	185	519
60 to 89 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	498
60 to 89 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Past Due [Line Items]
Past Due	0	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	1,355	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	1,068	1,305
60 to 89 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Past Due [Line Items]
Past Due	74	1,518
60 to 89 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	673	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	896	765
60 to 89 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Past Due [Line Items]
Past Due	109	385
90 Days or More Past Due [Member]
Financing Receivable, Past Due [Line Items]
Past Due	13,693	12,074
90 Days or More Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,459	2,223
90 Days or More Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,052	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	1,719	919
90 Days or More Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	642	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	2,213	1,756
90 Days or More Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Past Due [Line Items]
Past Due	614	1,601
90 Days or More Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Past Due [Line Items]
Past Due	2,454	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	1,416	706
90 Days or More Past Due [Member] | Consumer - other [Member]
Financing Receivable, Past Due [Line Items]
Past Due	$ 26	$ 9