LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 34,064	$ 28,859
Purchased Credit-Impaired	30,501	32,322	$ 53,271
Current	7,356,690	7,389,967
Total loans	7,421,255	7,451,148	7,185,999
Loans 90 days or more past due and still accruing	842	2,153
Nonaccrual loans	17,215	20,426
Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7,649	4,476
Purchased Credit-Impaired	11,639	13,281
Current	1,341,807	1,335,242
Total loans	1,361,095	1,352,999
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	2,461	3,373
Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,791	5,564
Purchased Credit-Impaired	10,391	10,168
Current	1,996,436	1,970,604
Total loans	2,011,618	1,986,336
Loans 90 days or more past due and still accruing	0	701
Nonaccrual loans	4,449	4,864
Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	147	147
Purchased Credit-Impaired	174	139	1,682
Current	253,925	247,864
Total loans	254,246	248,150	307,019
Loans 90 days or more past due and still accruing	0	147
Nonaccrual loans	147	0
Commercial construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Purchased Credit-Impaired	0	0
Current	141,505	124,068
Total loans	141,505	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	114,728	124,126
Total loans	114,728	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	2,195	0
Purchased Credit-Impaired	840	862
Current	363,156	374,842
Total loans	366,191	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	2,240	798
Purchased Credit-Impaired	0	0
Current	149,409	169,206
Total loans	151,649	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	977	998
Purchased Credit-Impaired	2,937	3,016
Current	25,683	25,170
Total loans	29,597	29,184
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	977	998
Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,979	4,552
Purchased Credit-Impaired	3,416	3,821	7,036
Current	1,216,146	1,199,506
Total loans	1,224,541	1,207,879	1,224,915
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	2,700	3,074
Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,447	4,372
Purchased Credit-Impaired	725	684	1,431
Current	310,202	364,100
Total loans	313,374	369,156	340,350
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	1,012	3,229
One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,996	4,152
Purchased Credit-Impaired	312	274	286
Current	796,683	808,651
Total loans	802,991	813,077	$ 910,719
Loans 90 days or more past due and still accruing	545	1,233
Nonaccrual loans	3,386	2,263
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,977	2,078
Purchased Credit-Impaired	11	18
Current	491,507	491,115
Total loans	493,495	493,211
Loans 90 days or more past due and still accruing	297	61
Nonaccrual loans	1,198	1,660
Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	666	1,722
Purchased Credit-Impaired	56	59
Current	155,503	155,473
Total loans	156,225	157,254
Loans 90 days or more past due and still accruing	0	11
Nonaccrual loans	87	215
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	19,016	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	5,940	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	454	117
30 to 59 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
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	2,195	0
30 to 59 Days Past Due [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,107	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	3,129	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	1,660	360
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,813	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	1,161	932
30 to 59 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	557	1,421
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,595	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	47	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	335	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	481	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	636	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	0	160
60 to 89 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	96	154
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	13,453	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	1,662	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	4,337	5,447
90 Days or More Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	147	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	977	998
90 Days or More Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,369	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	787	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	2,547	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	816	986
90 Days or More Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 13	$ 147