LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 8,835,368		$ 8,684,595	$ 7,822,519
Allowance for loan losses	97,801	$ 98,254	96,485	95,263	$ 93,875	$ 89,028
Net loans	$ 8,737,567		$ 8,588,110
Percent of total loans	100.00%		100.00%
Unearned loan fees in excess of unamortized costs	$ 43		$ (1,400)
Discount on acquired loans, net	21,300		25,700
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,463,303		$ 1,430,097
Percent of total loans	16.60%		16.40%
Commercial real estate - investment properties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,150,938		$ 2,131,059
Percent of total loans	24.30%		24.50%
Multifamily real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 399,814		$ 368,836	309,809
Allowance for loan losses	$ 4,283	4,344	$ 3,818	3,745	3,718	1,633
Percent of total loans	4.50%		4.20%
Commercial construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 190,532		$ 172,410
Percent of total loans	2.20%		2.00%
Multifamily Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 214,878		$ 184,630
Percent of total loans	2.40%		2.10%
One- to four-family construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 488,945		$ 534,678
Percent of total loans	5.50%		6.20%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 163,829		$ 188,508
Percent of total loans	1.90%		2.20%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 26,119		$ 27,278
Percent of total loans	0.30%		0.30%
Commercial business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,619,391		$ 1,483,614	1,358,149
Allowance for loan losses	$ 21,147	19,557	$ 19,438	17,948	19,141	18,311
Percent of total loans	18.30%		17.10%
Agricultural business, including secured by farmland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 390,505		$ 404,873	359,966
Allowance for loan losses	$ 3,895	3,691	$ 3,778	3,505	3,162	4,053
Percent of total loans	4.40%		4.70%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 947,475		$ 973,616	849,928
Allowance for loan losses	$ 4,569	$ 4,701	$ 4,714	$ 4,423	$ 3,932	$ 2,055
Percent of total loans	10.70%		11.20%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 566,792		$ 568,979
Percent of total loans	6.50%		6.60%
Consumer - other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 212,847		$ 216,017
Percent of total loans	2.40%		2.50%