LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 31,895	$ 28,859
Purchased Credit-Impaired	23,221	32,322	$ 38,674
Current	7,719,333	7,389,967
Total loans	7,774,449	7,451,148	7,398,637
Loans 90 days or more past due and still accruing	927	2,153
Nonaccrual loans	29,127	20,426
Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	6,930	4,476
Purchased Credit-Impaired	7,896	13,281
Current	1,354,304	1,335,242
Total loans	1,369,130	1,352,999
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	7,438	3,373
Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,136	5,564
Purchased Credit-Impaired	7,788	10,168
Current	1,981,220	1,970,604
Total loans	1,993,144	1,986,336
Loans 90 days or more past due and still accruing	53	701
Nonaccrual loans	4,194	4,864
Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,101	147
Purchased Credit-Impaired	173	139	258
Current	310,432	247,864
Total loans	311,706	248,150	266,883
Loans 90 days or more past due and still accruing	0	147
Nonaccrual loans	0	0
Commercial construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	223	0
Purchased Credit-Impaired	0	0
Current	156,818	124,068
Total loans	157,041	124,068
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	0
Purchased Credit-Impaired	0	0
Current	136,532	124,126
Total loans	136,532	124,126
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	0	0
Purchased Credit-Impaired	794	862
Current	398,567	374,842
Total loans	399,361	375,704
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	0	0
Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,617	798
Purchased Credit-Impaired	0	0
Current	156,767	169,206
Total loans	158,384	170,004
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	798	750
Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	928	998
Purchased Credit-Impaired	2,965	3,016
Current	23,202	25,170
Total loans	27,095	29,184
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	928	998
Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7,275	4,552
Purchased Credit-Impaired	2,608	3,821	4,264
Current	1,301,526	1,199,506
Total loans	1,311,409	1,207,879	1,187,848
Loans 90 days or more past due and still accruing	51	0
Nonaccrual loans	7,144	3,074
Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,381	4,372
Purchased Credit-Impaired	683	684	807
Current	335,868	364,100
Total loans	339,932	369,156	383,275
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	4,285	3,229
One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,270	4,152
Purchased Credit-Impaired	265	274	301
Current	866,021	808,651
Total loans	869,556	813,077	$ 846,899
Loans 90 days or more past due and still accruing	722	1,233
Nonaccrual loans	2,878	2,263
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,545	2,078
Purchased Credit-Impaired	5	18
Current	532,750	491,115
Total loans	535,300	493,211
Loans 90 days or more past due and still accruing	76	61
Nonaccrual loans	1,416	1,660
Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	489	1,722
Purchased Credit-Impaired	44	59
Current	165,326	155,473
Total loans	165,859	157,254
Loans 90 days or more past due and still accruing	25	11
Nonaccrual loans	46	215
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7,578	8,923
30 to 59 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	414	1,938
30 to 59 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	117
30 to 59 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,101	0
30 to 59 Days Past Due [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	223	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	0
30 to 59 Days Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	819	48
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	1,712	2,314
30 to 59 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,051	360
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	431	1,793
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	1,537	932
30 to 59 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	290	1,421
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,548	2,454
60 to 89 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,156	0
60 to 89 Days Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	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	0	0
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	371	647
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	1,244
60 to 89 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	628	249
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	220	160
60 to 89 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	173	154
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	19,769	17,482
90 Days or More Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,360	2,538
90 Days or More Past Due [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,136	5,447
90 Days or More Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	147
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	0	0
90 Days or More Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	798	750
90 Days or More Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	928	998
90 Days or More Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,192	1,591
90 Days or More Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,330	2,768
90 Days or More Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,211	2,110
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	788	986
90 Days or More Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 26	$ 147