LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Risk-Rate and Non-Risk Rated Loans by Grade and Other Characteristic) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	$ 9,305,357	$ 8,684,595
Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	1,580,650	1,430,097
Commer[Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	2,309,221	2,131,059
Multifamily Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	473,152	368,836
Commercial Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	210,668	172,410
Multifamily construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	233,610	184,630
One-to four-family construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	544,308	534,678
Residential Land and Land Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	154,688	188,508
Land and land development - commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	26,290	27,278
Commercial business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	1,693,824	1,483,614
Agricultural Business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	370,549	404,873
One- to four-family residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	945,622	973,616
Consumer secured by one- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	550,960	568,979
Consumer Borrower [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	211,815	216,017
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	9,130,662	8,557,839
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	764,600	590,900
Pass (Risk Ratings 1-5) [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	1,546,649	1,396,721
Pass (Risk Ratings 1-5) [Member] | Commer[Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	2,288,785	2,122,621
Pass (Risk Ratings 1-5) [Member] | Multifamily Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	472,856	368,262
Pass (Risk Ratings 1-5) [Member] | Commercial Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	198,986	159,167
Pass (Risk Ratings 1-5) [Member] | Multifamily construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	233,610	184,630
Pass (Risk Ratings 1-5) [Member] | One-to four-family construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	530,307	533,759
Pass (Risk Ratings 1-5) [Member] | Residential Land and Land Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	154,348	187,710
Pass (Risk Ratings 1-5) [Member] | Land and land development - commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	26,256	27,200
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	1,627,170	1,436,733
Pass (Risk Ratings 1-5) [Member] | Agricultural Business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	352,408	392,318
Pass (Risk Ratings 1-5) [Member] | One- to four-family residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	940,424	969,011
Pass (Risk Ratings 1-5) [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	547,388	564,001
Pass (Risk Ratings 1-5) [Member] | Consumer Borrower [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	211,475	215,706
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	61,189	31,162
Special Mention [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	4,198	6,963
Special Mention [Member] | Commer[Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	2,193	0
Special Mention [Member] | Multifamily Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Special Mention [Member] | Commercial Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	11,816
Special Mention [Member] | Multifamily construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Special Mention [Member] | One-to four-family construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	12,534	0
Special Mention [Member] | Residential Land and Land Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Special Mention [Member] | Land and land development - commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Special Mention [Member] | Commercial business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	31,012	7,661
Special Mention [Member] | Agricultural Business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	10,840	4,214
Special Mention [Member] | One- to four-family residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	409	499
Special Mention [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Special Mention [Member] | Consumer Borrower [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	3	9
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	113,448	95,507
Substandard [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	29,803	26,413
Substandard [Member] | Commer[Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	18,243	8,438
Substandard [Member] | Multifamily Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	296	574
Substandard [Member] | Commercial Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	11,682	1,427
Substandard [Member] | Multifamily construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Substandard [Member] | One-to four-family construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	1,467	919
Substandard [Member] | Residential Land and Land Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	340	798
Substandard [Member] | Land and land development - commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	34	78
Substandard [Member] | Commercial business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	35,584	39,133
Substandard [Member] | Agricultural Business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	7,301	8,341
Substandard [Member] | One- to four-family residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	4,789	4,106
Substandard [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	3,572	4,978
Substandard [Member] | Consumer Borrower [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	337	302
Doubtful [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	58	87
Doubtful [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | Commer[Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | Multifamily Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | Commercial Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | Multifamily construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | One-to four-family construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | Residential Land and Land Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | Land and land development - commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | Commercial business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	58	87
Doubtful [Member] | Agricultural Business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | One- to four-family residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | Consumer Borrower [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Commer[Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Multifamily Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Commercial Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Multifamily construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | One-to four-family construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Residential Land and Land Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Land and land development - commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Commercial business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Agricultural Business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | One- to four-family residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | Consumer Borrower [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	$ 0	$ 0