LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 20,624	$ 45,980
Purchased Credit-Impaired	12,944	21,310	$ 23,221
Current	7,788,951	7,531,594
Total loans	7,822,519	7,598,884	7,774,449
Loans 90 days or more past due and still accruing	1,887	1,486
Nonaccrual loans	13,919	25,543
Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,494	10,889
Purchased Credit-Impaired	5,369	7,682
Current	1,263,500	1,265,792
Total loans	1,271,363	1,284,363
Loans 90 days or more past due and still accruing	23	0
Nonaccrual loans	2,884	6,447
Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,697	5,833
Purchased Credit-Impaired	3,000	7,166
Current	1,939,096	1,924,424
Total loans	1,943,793	1,937,423
Loans 90 days or more past due and still accruing	405	0
Nonaccrual loans	844	4,199
Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	105
Purchased Credit-Impaired	142	169	173
Current	309,667	313,914
Total loans	309,809	314,188	311,706
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	0	0
Purchased Credit-Impaired	0	0
Current	154,071	148,435
Total loans	154,071	148,435
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	0	0
Multifamily Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	3,416
Purchased Credit-Impaired	0	0
Current	172,433	151,246
Total loans	172,433	154,662
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	1,586	5,915
Purchased Credit-Impaired	461	446
Current	496,502	408,966
Total loans	498,549	415,327
Loans 90 days or more past due and still accruing	0	298
Nonaccrual loans	1,297	0
Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,501	798
Purchased Credit-Impaired	0	0
Current	170,109	163,718
Total loans	171,610	164,516
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	798	798
Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Purchased Credit-Impaired	2,775	2,919
Current	19,607	21,664
Total loans	22,382	24,583
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	0	0
Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,401	4,555
Purchased Credit-Impaired	621	2,159	2,608
Current	1,353,127	1,273,180
Total loans	1,358,149	1,279,894	1,311,409
Loans 90 days or more past due and still accruing	87	18
Nonaccrual loans	2,921	3,406
Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,804	3,148
Purchased Credit-Impaired	398	565	683
Current	357,764	334,675
Total loans	359,966	338,388	339,932
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	1,645	6,132
One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,692	8,431
Purchased Credit-Impaired	111	136	265
Current	847,125	839,722
Total loans	849,928	848,289	$ 869,556
Loans 90 days or more past due and still accruing	1,076	1,085
Nonaccrual loans	1,827	3,264
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,865	2,130
Purchased Credit-Impaired	0	0
Current	535,278	520,801
Total loans	539,143	522,931
Loans 90 days or more past due and still accruing	282	85
Nonaccrual loans	1,611	1,239
Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	584	760
Purchased Credit-Impaired	67	68
Current	170,672	165,057
Total loans	171,323	165,885
Loans 90 days or more past due and still accruing	14	0
Nonaccrual loans	92	58
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,601	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	0	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	309	1,737
30 to 59 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	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	0	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	703	0
30 to 59 Days Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
30 to 59 Days Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,100	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	165	598
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	95	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	1,878	1,355
30 to 59 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	351	609
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,533	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	160	76
60 to 89 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	233	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	1,208	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	480	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	0	533
60 to 89 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	586	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	693	62
60 to 89 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	173	136
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	11,490	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	2,334	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	1,155	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	0	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	378	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	0	0
90 Days or More Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,821	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	1,639	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	2,011	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	1,294	713
90 Days or More Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 60	$ 15