LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Risk-Rated and Non-Risk Rated Loans by Grade and Other Characteristic) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	$ 7,774,449	$ 7,451,148	$ 7,398,637
Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,369,130	1,352,999
Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,993,144	1,986,336
Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	311,706	248,150	266,883
Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	157,041	124,068
Multifamily construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	136,532	124,126
One- to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	399,361	375,704
Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	158,384	170,004
Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	27,095	29,184
Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,311,409	1,207,879	1,187,848
Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	339,932	369,156	383,275
One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	869,556	813,077	$ 846,899
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	535,300	493,211
Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	165,859	157,254
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	7,612,423	7,282,408
Pass (Risk Ratings 1-5) [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,339,550	1,313,142
Pass (Risk Ratings 1-5) [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,978,603	1,948,822
Pass (Risk Ratings 1-5) [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	310,936	247,258
Pass (Risk Ratings 1-5) [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	157,041	124,068
Pass (Risk Ratings 1-5) [Member] | Multifamily construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	136,532	124,126
Pass (Risk Ratings 1-5) [Member] | One- to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	397,135	371,636
Pass (Risk Ratings 1-5) [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	147,874	167,764
Pass (Risk Ratings 1-5) [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	23,202	25,090
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,241,338	1,148,585
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	296,000	225,000
Pass (Risk Ratings 1-5) [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	317,808	356,656
Pass (Risk Ratings 1-5) [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	864,053	807,837
Pass (Risk Ratings 1-5) [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	532,907	490,877
Pass (Risk Ratings 1-5) [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	165,444	156,547
Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	37,120	77,691
Special mention [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	996	14,394
Special mention [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	3,601	23,846
Special mention [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Special mention [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Special mention [Member] | Multifamily construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Special mention [Member] | One- to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Special mention [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	9,374	0
Special mention [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Special mention [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	19,068	35,036
Special mention [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	3,390	3,335
Special mention [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	674	967
Special mention [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	5
Special mention [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	17	108
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	124,906	91,042
Substandard [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	28,584	25,463
Substandard [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	10,940	13,668
Substandard [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	770	892
Substandard [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Substandard [Member] | Multifamily construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Substandard [Member] | One- to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	2,226	4,068
Substandard [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,136	2,240
Substandard [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	3,893	4,094
Substandard [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	51,003	24,258
Substandard [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	18,734	9,165
Substandard [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	4,829	4,273
Substandard [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	2,393	2,327
Substandard [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	398	594
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	7
Doubtful [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Multifamily construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | One- to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	2
Doubtful [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	5
Loss [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Multifamily construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | One- to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	$ 0	$ 0