LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) (USD $)	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 3,290,201,000		$ 3,235,714,000	$ 3,212,257,000
Less allowance for loan losses	(76,853,000)	(77,128,000)	(77,491,000)	(80,221,000)	(81,544,000)	(82,912,000)
Total loans	3,213,348,000		3,158,223,000	3,132,036,000
Percent of total loans	100.00%		100.00%	100.00%
Unearned, unamortized loan fees (and costs)	8,900,000		8,900,000	9,700,000
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	500,812,000		489,581,000	477,621,000
Percent of total loans	15.20%		15.10%	14.90%
Commerical real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	595,896,000		583,641,000	613,965,000
Percent of total loans	18.10%		18.00%	19.10%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	137,027,000		137,504,000	130,319,000
Percent of total loans	4.20%		4.30%	4.10%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	25,629,000		30,229,000	23,808,000
Percent of total loans	0.80%		0.90%	0.70%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	39,787,000		22,581,000	18,132,000
Percent of total loans	1.20%		0.70%	0.60%
One-to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	191,003,000		160,815,000	157,301,000
Percent of total loans	5.80%		5.00%	4.90%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	86,037,000		77,010,000	83,185,000
Percent of total loans	2.60%		2.40%	2.60%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	11,228,000		13,982,000	11,451,000
Percent of total loans	0.30%		0.40%	0.40%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	639,840,000		618,049,000	600,046,000
Percent of total loans	19.40%		19.10%	18.70%
Agricultural business, including secured by farming [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	233,967,000		230,031,000	211,705,000
Percent of total loans	7.10%		7.10%	6.60%
One- to four-family real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	552,698,000		581,670,000	607,489,000
Percent of total loans	16.80%		18.00%	18.90%
Total consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	276,277,000		290,621,000
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	112,938,000		120,498,000	103,504,000
Percent of total loans	3.40%		3.70%	3.20%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans outstanding	$ 163,339,000		$ 170,123,000	$ 173,731,000
Percent of total loans	5.00%		5.30%	5.40%