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, 2019	Dec. 31, 2018	Sep. 30, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	$ 8,835,368	$ 8,684,595	$ 7,822,519
Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	1,463,303	1,430,097
Commercial real estate - investment properties [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	2,150,938	2,131,059
Multifamily real estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	399,814	368,836	309,809
Commercial construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	190,532	172,410
Multifamily Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	214,878	184,630
One- to four-family construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	488,945	534,678
Land and land development - residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	163,829	188,508
Land and land development - commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	26,119	27,278
Commercial business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	1,619,391	1,483,614	1,358,149
Agricultural Business/Farmland [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	390,505	404,873	359,966
One- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	947,475	973,616	$ 849,928
Consumer secured by one- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	566,792	568,979
Consumer Borrower [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	212,847	216,017
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	8,702,171	8,557,839
Pass (Risk Ratings 1-5) [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	1,430,712	1,396,721
Pass (Risk Ratings 1-5) [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	2,143,952	2,122,621
Pass (Risk Ratings 1-5) [Member] | Multifamily real estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	399,607	368,262
Pass (Risk Ratings 1-5) [Member] | Commercial construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	178,781	159,167
Pass (Risk Ratings 1-5) [Member] | Multifamily Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	214,878	184,630
Pass (Risk Ratings 1-5) [Member] | One- to four-family construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	483,968	533,759
Pass (Risk Ratings 1-5) [Member] | Land and land development - residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	163,187	187,710
Pass (Risk Ratings 1-5) [Member] | Land and land development - commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	26,084	27,200
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	1,564,854	1,436,733
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	740,600	590,900
Pass (Risk Ratings 1-5) [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	377,571	392,318
Pass (Risk Ratings 1-5) [Member] | One- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	942,696	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	563,517	564,001
Pass (Risk Ratings 1-5) [Member] | Consumer Borrower [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	212,364	215,706
Special mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	19,989	31,162
Special mention [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	5,010	6,963
Special mention [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	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	0	0
Special mention [Member] | Land and land development - residential [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	9,363	7,661
Special mention [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	5,191	4,214
Special mention [Member] | One- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	420	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	5	9
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	113,150	95,507
Substandard [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	27,581	26,413
Substandard [Member] | Commercial real estate - investment properties [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	6,986	8,438
Substandard [Member] | Multifamily real estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	207	574
Substandard [Member] | Commercial construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	11,751	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	4,977	919
Substandard [Member] | Land and land development - residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	642	798
Substandard [Member] | Land and land development - commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	35	78
Substandard [Member] | Commercial business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	45,116	39,133
Substandard [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	7,743	8,341
Substandard [Member] | One- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	4,359	4,106
Substandard [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	3,275	4,978
Substandard [Member] | Consumer Borrower [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	478	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] | Commercial real estate - investment properties [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] | Land and land development - residential [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/Farmland [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Doubtful [Member] | One- to four-family [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] | Commercial real estate - investment properties [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] | Land and land development - residential [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/Farmland [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans receivable	0	0
Loss [Member] | One- to four-family [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