LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 7,774,449		$ 7,451,148	$ 7,398,637
Allowance for loan losses	89,100	$ 88,586	85,997	84,220	$ 81,318	$ 78,008
Net loans	$ 7,685,349		$ 7,365,151
Percent of total loans	100.00%		100.00%
Unearned loan fees in excess of unamortized costs	$ 389		$ (5,800)
Discount on acquired loans, net	23,400		31,100
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,369,130		$ 1,352,999
Percent of total loans	17.60%		18.10%
Commercial real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,993,144		$ 1,986,336
Percent of total loans	25.60%		26.70%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 311,706		$ 248,150	266,883
Allowance for loan losses	$ 1,625	1,562	$ 1,360	1,436	1,515	4,195
Percent of total loans	4.00%		3.30%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 157,041		$ 124,068
Percent of total loans	2.00%		1.70%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 136,532		$ 124,126
Percent of total loans	1.80%		1.70%
One- to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 399,361		$ 375,704
Percent of total loans	5.10%		5.00%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 158,384		$ 170,004
Percent of total loans	2.00%		2.30%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 27,095		$ 29,184
Percent of total loans	0.40%		0.40%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,311,409		$ 1,207,879	1,187,848
Allowance for loan losses	$ 18,657	19,126	$ 16,533	16,507	17,758	13,856
Percent of total loans	16.90%		16.20%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 339,932		$ 369,156	383,275
Allowance for loan losses	$ 3,949	3,808	$ 2,967	2,833	2,891	3,645
Percent of total loans	4.40%		5.00%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 869,556		$ 813,077	846,899
Allowance for loan losses	$ 2,040	$ 2,010	$ 2,238	$ 2,190	$ 2,204	$ 4,732
Percent of total loans	11.20%		10.90%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 535,300		$ 493,211
Percent of total loans	6.90%		6.60%
Consumer - other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 165,859		$ 157,254
Percent of total loans	2.10%		2.10%