LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 7,684,732		$ 7,598,884	$ 7,551,563
Allowance for loan losses	93,875	$ 92,207	89,028	88,586	$ 86,527	$ 85,997
Net loans	$ 7,590,857		$ 7,509,856
Percent of total loans	100.00%		100.00%
Unearned loan fees in excess of unamortized costs	$ 1,387		$ (200)
Discount on acquired loans, net	18,100		21,100
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,256,730		$ 1,284,363
Percent of total loans	16.30%		16.90%
Commercial real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,920,790		$ 1,937,423
Percent of total loans	25.00%		25.50%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 330,384		$ 314,188	288,442
Allowance for loan losses	$ 3,718	2,592	$ 1,633	1,562	1,378	1,360
Percent of total loans	4.30%		4.10%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 166,089		$ 148,435
Percent of total loans	2.20%		2.00%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 147,576		$ 154,662
Percent of total loans	1.90%		2.00%
One- to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 480,591		$ 415,327
Percent of total loans	6.30%		5.50%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 163,335		$ 164,516
Percent of total loans	2.10%		2.20%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 22,849		$ 24,583
Percent of total loans	0.30%		0.30%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,312,424		$ 1,279,894	1,286,204
Allowance for loan losses	$ 19,141	19,885	$ 18,311	19,126	19,768	16,533
Percent of total loans	17.10%		16.80%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 336,709		$ 338,388	344,412
Allowance for loan losses	$ 3,162	2,999	$ 4,053	3,808	3,245	2,967
Percent of total loans	4.40%		4.40%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 840,470		$ 848,289	800,008
Allowance for loan losses	$ 3,932	$ 3,779	$ 2,055	$ 2,010	$ 1,974	$ 2,238
Percent of total loans	10.90%		11.20%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 536,007		$ 522,931
Percent of total loans	7.00%		6.90%
Consumer - other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 170,778		$ 165,885
Percent of total loans	2.20%		2.20%