LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 7,421,255	$ 7,451,148	$ 7,185,999
Allowance for loan losses	86,527	85,997	78,197	$ 78,008
Net loans	$ 7,334,728	$ 7,365,151
Percent of total loans	100.00%	100.00%
Unearned loan fees in excess of unamortized costs	$ 5,200	$ (5,800)
Discount on acquired loans, net	29,400	31,100
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,361,095	$ 1,352,999
Percent of total loans	18.40%	18.10%
Commercial real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,011,618	$ 1,986,336
Percent of total loans	27.10%	26.70%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 254,246	$ 248,150	307,019
Allowance for loan losses	$ 1,378	$ 1,360	2,853	4,195
Percent of total loans	3.40%	3.30%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 141,505	$ 124,068
Percent of total loans	1.90%	1.70%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 114,728	$ 124,126
Percent of total loans	1.60%	1.70%
One- to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 366,191	$ 375,704
Percent of total loans	4.90%	5.00%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 151,649	$ 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	$ 29,597	$ 29,184
Percent of total loans	0.40%	0.40%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,224,541	$ 1,207,879	1,224,915
Allowance for loan losses	$ 19,768	$ 16,533	15,118	13,856
Percent of total loans	16.50%	16.20%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 313,374	$ 369,156	340,350
Allowance for loan losses	$ 3,245	$ 2,967	4,282	3,645
Percent of total loans	4.20%	5.00%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 802,991	$ 813,077	910,719
Allowance for loan losses	$ 1,974	$ 2,238	$ 2,170	$ 4,732
Percent of total loans	10.80%	10.90%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 493,495	$ 493,211
Percent of total loans	6.70%	6.60%
Consumer - other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 156,225	$ 157,254
Percent of total loans	2.10%	2.10%