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, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	$ 8,684,595	$ 7,598,884
Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,430,097	1,284,363
Commer[Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	2,131,059	1,937,423
Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	368,836	314,188
Commercial Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	172,410	148,435
Multifamily construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	184,630	154,662
One-to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	534,678	415,327
Residential Land and Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	188,508	164,516
Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	27,278	24,583
Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,483,614	1,279,894
Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	404,873	338,388
One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	973,616	848,289
Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	568,979	522,931
Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	216,017	165,885
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	8,557,839	7,431,182
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	590,900	296,800
Pass (Risk Ratings 1-5) [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,396,721	1,246,125
Pass (Risk Ratings 1-5) [Member] | Commer[Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	2,122,621	1,918,940
Pass (Risk Ratings 1-5) [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	368,262	313,432
Pass (Risk Ratings 1-5) [Member] | Commercial Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	159,167	148,435
Pass (Risk Ratings 1-5) [Member] | Multifamily construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	184,630	154,662
Pass (Risk Ratings 1-5) [Member] | One-to four-family construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	533,759	411,802
Pass (Risk Ratings 1-5) [Member] | Residential Land and Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	187,710	153,073
Pass (Risk Ratings 1-5) [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	27,200	21,665
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,436,733	1,213,365
Pass (Risk Ratings 1-5) [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	392,318	321,110
Pass (Risk Ratings 1-5) [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	969,011	842,304
Pass (Risk Ratings 1-5) [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	564,001	520,675
Pass (Risk Ratings 1-5) [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	215,706	165,594
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	31,162	48,468
Special Mention [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	6,963	12,227
Special Mention [Member] | Commer[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	11,816	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] | Residential Land and Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	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	7,661	12,135
Special Mention [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	4,214	3,852
Special Mention [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	499	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	9	13
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	95,507	119,122
Substandard [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	26,413	26,011
Substandard [Member] | Commer[Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	8,438	9,365
Substandard [Member] | Multifamily Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	574	756
Substandard [Member] | Commercial Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	1,427	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	919	3,525
Substandard [Member] | Residential Land and Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	798	889
Substandard [Member] | Land and land development - commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	78	2,918
Substandard [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	39,133	54,282
Substandard [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	8,341	13,426
Substandard [Member] | One- to four-family residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	4,106	5,416
Substandard [Member] | Consumer secured by one- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	4,978	2,256
Substandard [Member] | Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	302	278
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	87	112
Doubtful [Member] | Owner-occupied Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | Commer[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] | Residential Land and Land Development [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	87	112
Doubtful [Member] | Agricultural Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Doubtful [Member] | One- to four-family residential [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] | Commer[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] | Residential Land and Land Development [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 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	0	0
Loss [Member] | One- to four-family residential [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