LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 28,859	$ 24,733
Carrying balance of purchased credit-impaired loans	32,322	58,600
Current	7,389,967	7,231,171
Loans receivable	7,451,148	7,314,504
Loans 90 Days or More Past Due and Accruing	2,153	952
Financing Receivable, Recorded Investment, Nonaccrual Status	20,426	14,270
Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,476	5,005
Carrying balance of purchased credit-impaired loans	13,281	24,261
Current	1,335,242	1,298,541
Loans receivable	1,352,999	1,327,807
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	3,373	1,235
Commerical real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,564	4,398
Carrying balance of purchased credit-impaired loans	10,168	16,724
Current	1,970,604	1,744,231
Loans receivable	1,986,336	1,765,353
Loans 90 Days or More Past Due and Accruing	701	0
Financing Receivable, Recorded Investment, Nonaccrual Status	4,864	2,516
Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	147	4
Carrying balance of purchased credit-impaired loans	139	1,994
Current	247,864	470,978
Loans receivable	248,150	472,976
Loans 90 Days or More Past Due and Accruing	147	0
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Carrying balance of purchased credit-impaired loans	0	0
Current	124,068	72,103
Loans receivable	124,068	72,103
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Multifamily construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	784
Carrying balance of purchased credit-impaired loans	0	0
Current	124,126	63,062
Loans receivable	124,126	63,846
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	0	2,686
Carrying balance of purchased credit-impaired loans	862	905
Current	374,842	274,878
Loans receivable	375,704	278,469
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	0	1,233
Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	798	747
Carrying balance of purchased credit-impaired loans	0	77
Current	169,206	125,949
Loans receivable	170,004	126,773
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	750	1,027
Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	998	96
Carrying balance of purchased credit-impaired loans	3,016	4,668
Current	25,170	28,415
Loans receivable	29,184	33,179
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	998	0
Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,552	3,042
Carrying balance of purchased credit-impaired loans	3,821	7,302
Current	1,199,506	1,197,600
Loans receivable	1,207,879	1,207,944
Loans 90 Days or More Past Due and Accruing	0	8
Financing Receivable, Recorded Investment, Nonaccrual Status	3,074	2,159
Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,372	1,330
Carrying balance of purchased credit-impaired loans	684	1,529
Current	364,100	373,672
Loans receivable	369,156	376,531
Loans 90 Days or More Past Due and Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	3,229	697
One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,152	5,304
Carrying balance of purchased credit-impaired loans	274	1,066
Current	808,651	946,263
Loans receivable	813,077	952,633
Loans 90 Days or More Past Due and Accruing	1,233	899
Financing Receivable, Recorded Investment, Nonaccrual Status	2,263	4,700
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,078	563
Carrying balance of purchased credit-impaired loans	18	40
Current	491,115	477,817
Loans receivable	493,211	478,420
Loans 90 Days or More Past Due and Accruing	61	4
Financing Receivable, Recorded Investment, Nonaccrual Status	1,660	565
Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,722	774
Carrying balance of purchased credit-impaired loans	59	34
Current	155,473	157,662
Loans receivable	157,254	158,470
Loans 90 Days or More Past Due and Accruing	11	41
Financing Receivable, Recorded Investment, Nonaccrual Status	215	138
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	8,923	12,725
30 to 59 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,938	3,981
30 to 59 Days Past Due [Member] | Commerical real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	117	1,763
30 to 59 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	4
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	771
30 to 59 Days Past Due [Member] | One-to four-family construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	2,466
30 to 59 Days Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	48	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	2,314	1,844
30 to 59 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	360	323
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,793	620
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	932	465
30 to 59 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,421	488
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,454	2,591
60 to 89 Days Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	139
60 to 89 Days Past Due [Member] | Commerical real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	132
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	13
60 to 89 Days Past Due [Member] | One-to four-family construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	220
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	96
60 to 89 Days Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	647	174
60 to 89 Days Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,244	729
60 to 89 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	249	873
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	160	60
60 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	154	155
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	17,482	9,417
90 Days or More Past Due [Member] | Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,538	885
90 Days or More Past Due [Member] | Commerical real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,447	2,503
90 Days or More Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	147	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	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	750	747
90 Days or More Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	998	0
90 Days or More Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,591	1,024
90 Days or More Past Due [Member] | Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,768	278
90 Days or More Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,110	3,811
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	986	38
90 Days or More Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 147	$ 131