LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 34,419	$ 38,202
Purchased Credit-Impaired	12,945	14,413	$ 18,063
Current	8,699,186	8,631,980
Total loans	8,746,550	8,684,595	7,684,732
Loans 90 days or more past due and still accruing	1,355	906
Nonaccrual loans	17,054	14,748
Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,644	3,527
Purchased Credit-Impaired	8,413	8,531
Current	1,421,938	1,418,039
Total loans	1,433,995	1,430,097
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	2,991	2,768
Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,760	1,523
Purchased Credit-Impaired	3,301	3,462
Current	2,111,245	2,126,074
Total loans	2,116,306	2,131,059
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	1,612	1,320
Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	91	317
Purchased Credit-Impaired	6	138	164
Current	402,144	368,381
Total loans	402,241	368,836	330,384
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	0	0
Commercial construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	605	1,427
Purchased Credit-Impaired	0	0
Current	172,326	170,983
Total loans	172,931	172,410
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	605	1,427
Multifamily Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Purchased Credit-Impaired	0	0
Current	189,160	184,630
Total loans	189,160	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, Recorded Investment, Past Due [Line Items]
Past Due	3,635	6,778
Purchased Credit-Impaired	0	137
Current	499,426	527,763
Total loans	503,061	534,678
Loans 90 days or more past due and still accruing	262	0
Nonaccrual loans	919	919
Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	690	1,248
Purchased Credit-Impaired	0	0
Current	186,490	187,260
Total loans	187,180	188,508
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	690	798
Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	78
Purchased Credit-Impaired	0	0
Current	27,470	27,200
Total loans	27,470	27,278
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	0	44
Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12,088	7,043
Purchased Credit-Impaired	526	1,028	1,454
Current	1,586,174	1,475,543
Total loans	1,598,788	1,483,614	1,312,424
Loans 90 days or more past due and still accruing	1	1
Nonaccrual loans	2,983	2,936
Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,520	3,462
Purchased Credit-Impaired	414	493	396
Current	378,871	400,918
Total loans	380,805	404,873	336,709
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	1,359	1,751
One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,541	8,887
Purchased Credit-Impaired	87	101	121
Current	938,989	964,628
Total loans	944,617	973,616	$ 840,470
Loans 90 days or more past due and still accruing	995	658
Nonaccrual loans	2,665	1,544
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,903	2,607
Purchased Credit-Impaired	113	432
Current	571,642	565,940
Total loans	575,658	568,979
Loans 90 days or more past due and still accruing	66	238
Nonaccrual loans	2,917	1,201
Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	942	1,305
Purchased Credit-Impaired	85	91
Current	213,311	214,621
Total loans	214,338	216,017
Loans 90 days or more past due and still accruing	31	9
Nonaccrual loans	313	40
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13,084	18,270
30 to 59 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,024	785
30 to 59 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	148	91
30 to 59 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	91	317
30 to 59 Days Past Due [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
30 to 59 Days Past Due [Member] | Multifamily Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
30 to 59 Days Past Due [Member] | One- to four-family construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	4,781
30 to 59 Days Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	450
30 to 59 Days Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	34
30 to 59 Days Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	8,982	3,982
30 to 59 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	131	343
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	826	5,440
30 to 59 Days Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,324	1,136
30 to 59 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	558	911
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,496	7,858
60 to 89 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	146	519
60 to 89 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	498
60 to 89 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] | Multifamily Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] | One- to four-family construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,454	1,078
60 to 89 Days Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60 to 89 Days Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	895	1,305
60 to 89 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	30	1,518
60 to 89 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,496	1,790
60 to 89 Days Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	346	765
60 to 89 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	129	385
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	15,839	12,074
90 Days or More Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,474	2,223
90 Days or More Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,612	934
90 Days or More Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
90 Days or More Past Due [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	605	1,427
90 Days or More Past Due [Member] | Multifamily Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
90 Days or More Past Due [Member] | One- to four-family construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,181	919
90 Days or More Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	690	798
90 Days or More Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	44
90 Days or More Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,211	1,756
90 Days or More Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,359	1,601
90 Days or More Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,219	1,657
90 Days or More Past Due [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,233	706
90 Days or More Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 255	$ 9