LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 3,240,321,000	$ 3,235,714,000	$ 3,229,662,000
Less allowance for loan losses	(77,128,000)	(77,491,000)	(81,544,000)	(82,912,000)
Total loans	3,163,193,000	3,158,223,000	3,148,118,000
Percent of total loans	100.00%	100.00%	100.00%
Unearned, unamortized loan fees (and costs)	9,000,000	9,000,000	10,000,000
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	497,442,000	489,581,000	468,318,000
Percent of total loans	15.30%	15.10%	14.50%
Commerical real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	602,761,000	583,641,000	612,617,000
Percent of total loans	18.60%	18.00%	19.00%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	134,290,000	137,504,000	132,306,000
Percent of total loans	4.10%	4.30%	4.10%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	34,762,000	30,229,000	40,276,000
Percent of total loans	1.10%	0.90%	1.20%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	34,147,000	22,581,000	20,654,000
Percent of total loans	1.10%	0.70%	0.60%
One-to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	171,876,000	160,815,000	148,717,000
Percent of total loans	5.30%	5.00%	4.60%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	78,446,000	77,010,000	89,329,000
Percent of total loans	2.40%	2.40%	2.70%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	12,477,000	13,982,000	12,044,000
Percent of total loans	0.40%	0.40%	0.40%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	619,478,000	618,049,000	609,497,000
Percent of total loans	19.10%	19.10%	18.90%
Agricultural business, including secured by farming [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	210,225,000	230,031,000	188,955,000
Percent of total loans	6.50%	7.10%	5.90%
One- to four-family real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	566,730,000	581,670,000	619,511,000
Percent of total loans	17.50%	18.00%	19.20%
Total consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	277,687,000	290,621,000
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	112,382,000	120,498,000	106,978,000
Percent of total loans	3.50%	3.70%	3.30%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 165,305,000	$ 170,123,000	$ 180,460,000
Percent of total loans	5.10%	5.30%	5.60%