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, 2019	Dec. 31, 2018	Jun. 30, 2018
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	$ 8,746,550	$ 8,684,595	$ 7,684,732
Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,433,995	1,430,097
Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	2,116,306	2,131,059
Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	402,241	368,836	330,384
Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	172,931	172,410
Multifamily Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	189,160	184,630
One- to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	503,061	534,678
Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	187,180	188,508
Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	27,470	27,278
Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,598,788	1,483,614	1,312,424
Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	380,805	404,873	336,709
One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	944,617	973,616	$ 840,470
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	575,658	568,979
Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	214,338	216,017
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	8,628,532	8,557,839
Pass (Risk Ratings 1-5) [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,409,358	1,396,721
Pass (Risk Ratings 1-5) [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	2,107,989	2,122,621
Pass (Risk Ratings 1-5) [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	401,810	368,262
Pass (Risk Ratings 1-5) [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	160,616	159,167
Pass (Risk Ratings 1-5) [Member] | Multifamily Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	189,160	184,630
Pass (Risk Ratings 1-5) [Member] | One- to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	502,142	533,759
Pass (Risk Ratings 1-5) [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	186,490	187,710
Pass (Risk Ratings 1-5) [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	27,434	27,200
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,549,519	1,436,733
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	744,800	590,900
Pass (Risk Ratings 1-5) [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	368,172	392,318
Pass (Risk Ratings 1-5) [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	940,607	969,011
Pass (Risk Ratings 1-5) [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	571,458	564,001
Pass (Risk Ratings 1-5) [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	213,777	215,706
Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	37,261	31,162
Special mention [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	7,880	6,963
Special mention [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
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	11,816
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	0	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	21,832	7,661
Special mention [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	7,109	4,214
Special mention [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	434	499
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	6	9
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	80,679	95,507
Substandard [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	16,757	26,413
Substandard [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	8,317	8,438
Substandard [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	431	574
Substandard [Member] | Commercial construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	12,315	1,427
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	919	919
Substandard [Member] | Land and land development - residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	690	798
Substandard [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	36	78
Substandard [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	27,359	39,133
Substandard [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	5,524	8,341
Substandard [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	3,576	4,106
Substandard [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	4,200	4,978
Substandard [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	555	302
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	78	87
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	78	87
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