LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 8,684,595	$ 7,598,884
Allowance for loan losses	96,485	89,028	$ 85,997	$ 78,008
Total loans, net	$ 8,588,110	$ 7,509,856
Percent of total loans	100.00%	100.00%
Unearned loan fees in excess of unamortized costs	$ 1,375	$ (200)
Discount on acquired loans, net	25,700	21,100
Related party loans	3,500	3,500
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 1,430,097	$ 1,284,363
Percent of total loans	16.40%	16.90%
Commercial real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 2,131,059	$ 1,937,423
Percent of total loans	24.50%	25.50%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 368,836	$ 314,188
Allowance for loan losses	$ 3,818	$ 1,633	1,360	4,195
Percent of total loans	4.20%	4.10%
Commercial Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 172,410	$ 148,435
Percent of total loans	2.00%	2.00%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 184,630	$ 154,662
Percent of total loans	2.10%	2.00%
One-to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 534,678	$ 415,327
Percent of total loans	6.20%	5.50%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 188,508	$ 164,516
Percent of total loans	2.20%	2.20%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 27,278	$ 24,583
Percent of total loans	0.30%	0.30%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 1,483,614	$ 1,279,894
Allowance for loan losses	$ 19,438	$ 18,311	16,533	13,856
Percent of total loans	17.10%	16.80%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 404,873	$ 338,388
Allowance for loan losses	$ 3,778	$ 4,053	2,967	3,645
Percent of total loans	4.70%	4.40%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 973,616	$ 848,289
Allowance for loan losses	$ 4,714	$ 2,055	$ 2,238	$ 4,732
Percent of total loans	11.20%	11.20%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 568,979	$ 522,931
Percent of total loans	6.60%	6.90%
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 216,017	$ 165,885
Percent of total loans	2.50%	2.20%