LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 4,372,594		$ 3,833,820	$ 3,806,695
Allowance for loan losses	77,320	$ 77,329	75,907	74,331	$ 74,310	$ 74,258
Total loans, net	$ 4,295,274		$ 3,757,913
Percent of total loans	100.00%		100.00%
Unearned loan fees in excess of unamortized costs	$ 10,000		$ (5,800)
Discount on acquired loans, net	4,300		148
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 635,146		$ 546,783
Percent of total loans	14.50%		14.30%
Commerical real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 1,062,418		$ 856,942
Percent of total loans	24.30%		22.30%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 198,874		$ 167,524	183,944
Allowance for loan losses	$ 4,363	4,273	$ 4,562	4,892	5,765	5,306
Percent of total loans	4.60%		4.40%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 47,490		$ 17,337
Percent of total loans	1.10%		0.40%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 72,987		$ 60,193
Percent of total loans	1.70%		1.60%
One- to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 246,715		$ 219,889
Percent of total loans	5.60%		5.70%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 111,091		$ 102,435
Percent of total loans	2.50%		2.70%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 15,517		$ 11,152
Percent of total loans	0.40%		0.30%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 812,070		$ 723,964	728,088
Allowance for loan losses	$ 12,765	13,184	$ 12,043	12,677	12,014	11,773
Percent of total loans	18.60%		18.90%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 242,556		$ 238,499	240,048
Allowance for loan losses	$ 2,533	2,679	$ 2,821	2,947	2,824	2,841
Percent of total loans	5.50%		6.20%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 536,325		$ 539,894	527,271
Allowance for loan losses	$ 7,937	$ 8,542	$ 8,447	$ 9,136	$ 9,270	$ 11,486
Percent of total loans	12.30%		14.10%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 250,029		$ 222,205
Percent of total loans	5.70%		5.80%
Consumer - other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 141,376		$ 127,003
Percent of total loans	3.20%		3.30%