LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Age Analysis of Company's Past Due Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 39,068	$ 24,733
Purchased Credit-Impaired	53,271	58,600	$ 5,674
Current	7,093,660	7,231,171
Total loans	7,185,999	7,314,504	4,105,399
Loans 90 days or more past due and still accruing	1,290	952
Nonaccrual loans	14,325	14,270
Commerical real estate - owner-occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	3,794	5,005
Purchased Credit-Impaired	20,135	24,261
Current	1,304,105	1,298,541
Total loans	1,328,034	1,327,807
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	1,555	1,235
Commerical real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	10,942	4,398
Purchased Credit-Impaired	18,161	16,724
Current	1,776,140	1,744,231
Total loans	1,805,243	1,765,353
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	2,590	2,516
Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	324	4
Purchased Credit-Impaired	1,682	1,994	0
Current	305,013	470,978
Total loans	307,019	472,976	208,687
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	87,711	72,103
Total loans	87,711	72,103
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	784
Purchased Credit-Impaired	0	0
Current	79,737	63,062
Total loans	79,737	63,846
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	3,457	2,686
Purchased Credit-Impaired	901	905
Current	292,990	274,878
Total loans	297,348	278,469
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	0	1,233
Land and land development - residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	750	747
Purchased Credit-Impaired	76	77
Current	142,015	125,949
Total loans	142,841	126,773
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	1,222	1,027
Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,027	96
Purchased Credit-Impaired	2,948	4,668
Current	20,518	28,415
Total loans	24,493	33,179
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	1,028	0
Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,584	3,042
Purchased Credit-Impaired	7,036	7,302	0
Current	1,215,295	1,197,600
Total loans	1,224,915	1,207,944	776,579
Loans 90 days or more past due and still accruing	0	8
Nonaccrual loans	1,558	2,159
Agricultural business, including secured by farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	5,873	1,330
Purchased Credit-Impaired	1,431	1,529	0
Current	333,046	373,672
Total loans	340,350	376,531	208,635
Loans 90 days or more past due and still accruing	0	0
Nonaccrual loans	663	697
One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	7,432	5,304
Purchased Credit-Impaired	286	1,066	1,092
Current	903,001	946,263
Total loans	910,719	952,633	$ 543,004
Loans 90 days or more past due and still accruing	1,039	899
Nonaccrual loans	4,803	4,700
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,822	563
Purchased Credit-Impaired	229	40
Current	479,539	477,817
Total loans	481,590	478,420
Loans 90 days or more past due and still accruing	147	4
Nonaccrual loans	743	565
Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,063	774
Purchased Credit-Impaired	386	34
Current	154,550	157,662
Total loans	155,999	158,470
Loans 90 days or more past due and still accruing	104	41
Nonaccrual loans	163	138
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	26,374	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	2,906	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	8,484	1,763
30 to 59 Days Past Due [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	324	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	3,457	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	0	0
30 to 59 Days Past Due [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,027	0
30 to 59 Days Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	864	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	4,238	323
30 to 59 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,920	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	1,436	465
30 to 59 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	718	488
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,111	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	362	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	469	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	972	729
60 to 89 Days Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	27	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	115	60
60 to 89 Days Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	166	155
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	10,583	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	526	885
90 Days or More Past Due [Member] | Commerical real estate - investment properties [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	2,458	2,503
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	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	0	0
90 Days or More Past Due [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	1,251	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	663	278
90 Days or More Past Due [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	4,485	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	271	38
90 Days or More Past Due [Member] | Consumer - other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 179	$ 131