LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 38,202	$ 45,980
Carrying balance of purchased credit-impaired loans	14,413	21,310
Current	8,631,980	7,531,594
Loans receivable	8,684,595	7,598,884
Loans 90 Days or More Past Due and Accruing	906	1,486
Financing Receivable, Recorded Investment, Nonaccrual Status	14,748	25,543
Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,527	10,889
Carrying balance of purchased credit-impaired loans	8,531	7,682
Current	1,418,039	1,265,792
Loans receivable	1,430,097	1,284,363
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	2,768	6,447
Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,523	5,833
Carrying balance of purchased credit-impaired loans	3,462	7,166
Current	2,126,074	1,924,424
Loans receivable	2,131,059	1,937,423
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	1,320	4,199
Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	317	105
Carrying balance of purchased credit-impaired loans	138	169
Current	368,381	313,914
Loans receivable	368,836	314,188
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,427	0
Carrying balance of purchased credit-impaired loans	0	0
Current	170,983	148,435
Loans receivable	172,410	148,435
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	1,427	0
Multifamily construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	3,416
Carrying balance of purchased credit-impaired loans	0	0
Current	184,630	151,246
Loans receivable	184,630	154,662
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
One-to four-family construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	6,778	5,915
Carrying balance of purchased credit-impaired loans	137	446
Current	527,763	408,966
Loans receivable	534,678	415,327
Loans 90 Days or More Past Due and Accruing	0	298
Financing Receivable, Recorded Investment, Nonaccrual Status	919	0
Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,248	798
Carrying balance of purchased credit-impaired loans	0	0
Current	187,260	163,718
Loans receivable	188,508	164,516
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	798	798
Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	78	0
Carrying balance of purchased credit-impaired loans	0	2,919
Current	27,200	21,664
Loans receivable	27,278	24,583
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	44	0
Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7,043	4,555
Carrying balance of purchased credit-impaired loans	1,028	2,159
Current	1,475,543	1,273,180
Loans receivable	1,483,614	1,279,894
Loans 90 Days or More Past Due and Accruing	1	18
Financing Receivable, Recorded Investment, Nonaccrual Status	2,936	3,406
Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,462	3,148
Carrying balance of purchased credit-impaired loans	493	565
Current	400,918	334,675
Loans receivable	404,873	338,388
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	1,751	6,132
One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	8,887	8,431
Carrying balance of purchased credit-impaired loans	101	136
Current	964,628	839,722
Loans receivable	973,616	848,289
Loans 90 Days or More Past Due and Accruing	658	1,085
Financing Receivable, Recorded Investment, Nonaccrual Status	1,544	3,264
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,607	2,130
Carrying balance of purchased credit-impaired loans	432	0
Current	565,940	520,801
Loans receivable	568,979	522,931
Loans 90 Days or More Past Due and Accruing	238	85
Financing Receivable, Recorded Investment, Nonaccrual Status	1,201	1,239
Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,305	760
Carrying balance of purchased credit-impaired loans	91	68
Current	214,621	165,057
Loans receivable	216,017	165,885
Loans 90 Days or More Past Due and Accruing	9	0
Financing Receivable, Recorded Investment, Nonaccrual Status	40	58
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	18,270	24,084
30 to 59 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	785	5,323
30 to 59 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	91	1,737
30 to 59 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	317	105
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	3,416
30 to 59 Days Past Due [Member] | One-to four-family construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,781	4,892
30 to 59 Days Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	450	0
30 to 59 Days Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	34	0
30 to 59 Days Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,982	1,574
30 to 59 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	343	598
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,440	4,475
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	1,136	1,355
30 to 59 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	911	609
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7,858	3,177
60 to 89 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	519	76
60 to 89 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	498	0
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	1,078	725
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	1,305	404
60 to 89 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,518	533
60 to 89 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,790	1,241
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	765	62
60 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	385	136
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	12,074	18,719
90 Days or More Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,223	5,490
90 Days or More Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	934	4,096
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	1,427	0
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	919	298
90 Days or More Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	798	798
90 Days or More Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	44	0
90 Days or More Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,756	2,577
90 Days or More Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,601	2,017
90 Days or More Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,657	2,715
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	706	713
90 Days or More Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 9	$ 15