LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 7,398,637		$ 7,314,504	$ 4,372,594
Allowance for loan losses	84,220	$ 81,318	78,008	77,320	$ 77,329	$ 75,907
Net loans	$ 7,314,417		$ 7,236,496
Percent of total loans	100.00%		100.00%
Unearned loan fees in excess of unamortized costs	$ 5,600		$ (5,500)
Discount on acquired loans, net	34,900		43,700
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,340,577		$ 1,327,807
Percent of total loans	18.10%		18.20%
Commerical real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,918,639		$ 1,765,353
Percent of total loans	25.90%		24.10%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 266,883		$ 472,976	198,874
Allowance for loan losses	$ 1,436	1,515	$ 4,195	4,363	4,273	4,562
Percent of total loans	3.60%		6.50%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 135,487		$ 72,103
Percent of total loans	1.80%		1.00%
Multifamily construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 105,669		$ 63,846
Percent of total loans	1.40%		0.90%
One- to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 363,586		$ 278,469
Percent of total loans	4.90%		3.80%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 162,029		$ 126,773
Percent of total loans	2.20%		1.70%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 30,556		$ 33,179
Percent of total loans	0.40%		0.50%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,187,848		$ 1,207,944	812,070
Allowance for loan losses	$ 16,507	17,758	$ 13,856	12,765	13,184	12,043
Percent of total loans	16.10%		16.50%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 383,275		$ 376,531	242,556
Allowance for loan losses	$ 2,833	2,891	$ 3,645	2,533	2,679	2,821
Percent of total loans	5.20%		5.10%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 846,899		$ 952,633	536,325
Allowance for loan losses	$ 2,190	$ 2,204	$ 4,732	$ 7,937	$ 8,542	$ 8,447
Percent of total loans	11.50%		13.00%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 497,643		$ 478,420
Percent of total loans	6.70%		6.50%
Consumer - other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 159,546		$ 158,470
Percent of total loans	2.20%		2.20%