LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Risk-Rated and Non-Risk Rated Loans by Grade and Other Characteristic) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	$ 7,684,732	$ 7,598,884	$ 7,551,563
Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,256,730	1,284,363
Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,920,790	1,937,423
Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	330,384	314,188	288,442
Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	166,089	148,435
Multifamily construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	147,576	154,662
One- to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	480,591	415,327
Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	163,335	164,516
Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	22,849	24,583
Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,312,424	1,279,894	1,286,204
Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	336,709	338,388	344,412
One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	840,470	848,289	$ 800,008
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	536,007	522,931
Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	170,778	165,885
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	7,539,420	7,431,182
Pass (Risk Ratings 1-5) [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,223,858	1,246,125
Pass (Risk Ratings 1-5) [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,912,516	1,918,940
Pass (Risk Ratings 1-5) [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	329,887	313,432
Pass (Risk Ratings 1-5) [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	166,089	148,435
Pass (Risk Ratings 1-5) [Member] | Multifamily construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	147,576	154,662
Pass (Risk Ratings 1-5) [Member] | One- to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	478,361	411,802
Pass (Risk Ratings 1-5) [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	152,083	153,073
Pass (Risk Ratings 1-5) [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	20,047	21,665
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,247,794	1,213,365
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	558,400	296,800
Pass (Risk Ratings 1-5) [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	323,137	321,110
Pass (Risk Ratings 1-5) [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	834,766	842,304
Pass (Risk Ratings 1-5) [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	532,758	520,675
Pass (Risk Ratings 1-5) [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	170,548	165,594
Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	46,885	48,468
Special mention [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	13,613	12,227
Special mention [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	9,118
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	10,453	10,554
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	17,320	12,135
Special mention [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	4,952	3,852
Special mention [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	536	569
Special mention [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Special mention [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	11	13
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	98,322	119,122
Substandard [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	19,259	26,011
Substandard [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	8,274	9,365
Substandard [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	497	756
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	2,230	3,525
Substandard [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	799	889
Substandard [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	2,802	2,918
Substandard [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	47,205	54,282
Substandard [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	8,620	13,426
Substandard [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	5,168	5,416
Substandard [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	3,249	2,256
Substandard [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	219	278
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	105	112
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	105	112
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	0
Doubtful [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
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