LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date	Dec. 31, 2019
Total loans outstanding	$ 9,305,357	$ 8,684,595
Allowance for loan losses	100,559	96,485	$ 89,028	$ 85,997
Total loans, net	$ 9,204,798	$ 8,588,110
Percent of total loans	100.00%	100.00%
Unearned loan fees in excess of unamortized costs	$ 438	$ (1,400)
Discount on acquired loans, net	25,000	25,700
Related party loans	3,300	3,500
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 1,580,650	$ 1,430,097
Percent of total loans	17.00%	16.40%
Commercial real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 2,309,221	$ 2,131,059
Percent of total loans	24.80%	24.50%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 473,152	$ 368,836
Allowance for loan losses	$ 4,754	$ 3,818	1,633	1,360
Percent of total loans	5.10%	4.20%
Commercial Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 210,668	$ 172,410
Percent of total loans	2.30%	2.00%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 233,610	$ 184,630
Percent of total loans	2.50%	2.10%
One-to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 544,308	$ 534,678
Percent of total loans	5.80%	6.20%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 154,688	$ 188,508
Percent of total loans	1.70%	2.20%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 26,290	$ 27,278
Percent of total loans	0.30%	0.30%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 1,693,824	$ 1,483,614
Allowance for loan losses	$ 23,370	$ 19,438	18,311	16,533
Percent of total loans	18.20%	17.10%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 370,549	$ 404,873
Allowance for loan losses	$ 4,120	$ 3,778	4,053	2,967
Percent of total loans	4.00%	4.70%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 945,622	$ 973,616
Allowance for loan losses	$ 4,136	$ 4,714	$ 2,055	$ 2,238
Percent of total loans	10.10%	11.20%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 550,960	$ 568,979
Percent of total loans	5.90%	6.60%
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 211,815	$ 216,017
Percent of total loans	2.30%	2.50%