LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 4,246,476		$ 3,833,820	$ 3,762,599
Allowance for loan losses	77,329	$ 75,365	75,907	74,310	$ 74,371	$ 74,258
Total loans, net	$ 4,169,147		$ 3,757,913
Loans Receivable, Percent of Total	100.00%		100.00%
Unearned loan (fees) in excess of unamortized costs	$ 10,100		$ 5,800
Discount on acquired loans, net	4,600		148
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 616,324		$ 546,783
Loans Receivable, Percent of Total	14.50%		14.30%
Commerical real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 996,714		$ 856,942
Loans Receivable, Percent of Total	23.50%		22.30%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 205,276		$ 167,524	188,792
Allowance for loan losses	$ 4,273	4,401	$ 4,562	5,765	5,652	5,306
Loans Receivable, Percent of Total	4.80%		4.40%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 45,137		$ 17,337
Loans Receivable, Percent of Total	1.10%		0.40%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 60,075		$ 60,193
Loans Receivable, Percent of Total	1.40%		1.60%
One- to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 230,554		$ 219,889
Loans Receivable, Percent of Total	5.40%		5.70%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 105,146		$ 102,435
Loans Receivable, Percent of Total	2.50%		2.70%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 16,419		$ 11,152
Loans Receivable, Percent of Total	0.40%		0.30%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 811,623		$ 723,964	735,128
Allowance for loan losses	$ 13,184	12,892	$ 12,043	12,014	11,363	11,773
Loans Receivable, Percent of Total	19.10%		18.90%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 230,964		$ 238,499	245,742
Allowance for loan losses	$ 2,679	3,732	$ 2,821	2,824	2,636	2,841
Loans Receivable, Percent of Total	5.40%		6.20%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 542,961		$ 539,894	558,744
Allowance for loan losses	$ 8,542	$ 8,141	$ 8,447	$ 9,270	$ 10,913	$ 11,486
Loans Receivable, Percent of Total	12.80%		14.10%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 244,216		$ 222,205
Loans Receivable, Percent of Total	5.80%		5.80%
Consumer - other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 141,067		$ 127,003
Loans Receivable, Percent of Total	3.30%		3.30%