LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 7,822,519		$ 7,598,884	$ 7,774,449
Allowance for loan losses	95,263	$ 93,875	89,028	89,100	$ 88,586	$ 85,997
Net loans	$ 7,727,256		$ 7,509,856
Percent of total loans	100.00%		100.00%
Unearned loan fees in excess of unamortized costs	$ 1,676		$ (200)
Discount on acquired loans, net	15,400		21,100
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,271,363		$ 1,284,363
Percent of total loans	16.20%		16.90%
Commercial real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,943,793		$ 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	$ 309,809		$ 314,188	311,706
Allowance for loan losses	$ 3,745	3,718	$ 1,633	1,625	1,562	1,360
Percent of total loans	3.90%		4.10%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 154,071		$ 148,435
Percent of total loans	2.00%		2.00%
Multifamily Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 172,433		$ 154,662
Percent of total loans	2.20%		2.00%
One- to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 498,549		$ 415,327
Percent of total loans	6.40%		5.50%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 171,610		$ 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	$ 22,382		$ 24,583
Percent of total loans	0.30%		0.30%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,358,149		$ 1,279,894	1,311,409
Allowance for loan losses	$ 17,948	19,141	$ 18,311	18,657	19,126	16,533
Percent of total loans	17.40%		16.80%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 359,966		$ 338,388	339,932
Allowance for loan losses	$ 3,505	3,162	$ 4,053	3,949	3,808	2,967
Percent of total loans	4.60%		4.40%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 849,928		$ 848,289	869,556
Allowance for loan losses	$ 4,423	$ 3,932	$ 2,055	$ 2,040	$ 2,010	$ 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	$ 539,143		$ 522,931
Percent of total loans	6.90%		6.90%
Consumer - other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 171,323		$ 165,885
Percent of total loans	2.20%		2.20%