LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 7,556,046	$ 7,598,884	$ 7,421,255
Allowance for loan losses	92,207	89,028	86,527	$ 85,997
Net loans	$ 7,463,839	$ 7,509,856
Percent of total loans	100.00%	100.00%
Unearned loan fees in excess of unamortized costs	$ 731	$ (200)
Discount on acquired loans, net	19,400	21,100
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,278,814	$ 1,284,363
Percent of total loans	16.90%	16.90%
Commercial real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,876,937	$ 1,937,423
Percent of total loans	24.80%	25.50%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 321,039	$ 314,188	254,246
Allowance for loan losses	$ 2,592	$ 1,633	1,378	1,360
Percent of total loans	4.20%	4.10%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 163,314	$ 148,435
Percent of total loans	2.20%	2.00%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 159,108	$ 154,662
Percent of total loans	2.10%	2.00%
One- to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 434,204	$ 415,327
Percent of total loans	5.70%	5.50%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 167,783	$ 164,516
Percent of total loans	2.20%	2.20%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 24,331	$ 24,583
Percent of total loans	0.30%	0.30%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,296,691	$ 1,279,894	1,224,541
Allowance for loan losses	$ 19,885	$ 18,311	19,768	16,533
Percent of total loans	17.20%	16.80%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 307,243	$ 338,388	313,374
Allowance for loan losses	$ 2,999	$ 4,053	3,245	2,967
Percent of total loans	4.20%	4.40%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 833,598	$ 848,289	802,991
Allowance for loan losses	$ 3,779	$ 2,055	$ 1,974	$ 2,238
Percent of total loans	11.00%	11.20%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 522,826	$ 522,931
Percent of total loans	6.90%	6.90%
Consumer - other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 170,158	$ 165,885
Percent of total loans	2.30%	2.20%