LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Loans by Type) (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 9,285,744,000	$ 9,305,357,000	$ 8,692,657,000
Allowance for credit losses - loans	(130,488,000)	(100,559,000)	(97,308,000)	$ (96,485,000)
Net loans	9,155,256,000	$ 9,204,798,000
Percent of total loans		100.00%
Unearned loan fees in excess of unamortized costs	451,000	$ (438,000)
Discount on acquired loans, net	22,200,000	25,000,000.0
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,024,089,000	$ 1,580,650,000
Percent of total loans		17.00%
Commercial real estate - investment properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,007,537,000	$ 2,309,221,000
Percent of total loans		24.80%
Multifamily real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	400,206,000	$ 473,152,000	387,142,000
Allowance for credit losses - loans	(2,805,000)	$ (4,754,000)	(4,020,000)	(3,818,000)
Percent of total loans		5.10%
Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	205,476,000	$ 210,668,000
Percent of total loans		2.30%
Multifamily construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	250,410,000	$ 233,610,000
Percent of total loans		2.50%
One- to four- family construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	534,956,000	$ 544,308,000
Percent of total loans		5.80%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	232,506,000	$ 154,688,000
Percent of total loans		1.70%
Land and land development - commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable		$ 26,290,000
Percent of total loans		0.30%
Commercial business
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,357,817,000	$ 1,693,824,000	1,524,298,000
Allowance for credit losses - loans	(31,648,000)	$ (23,370,000)	(18,662,000)	(19,438,000)
Percent of total loans		18.20%
Small business scored
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	807,539,000
Agricultural business, including secured by farmland
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	330,257,000	$ 370,549,000	373,322,000
Allowance for credit losses - loans	(4,513,000)	$ (4,120,000)	(3,596,000)	(3,778,000)
Percent of total loans		4.00%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	881,387,000	$ 945,622,000	967,581,000
Allowance for credit losses - loans	(11,884,000)	$ (4,136,000)	$ (4,711,000)	$ (4,714,000)
Percent of total loans		10.20%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	521,618,000	$ 550,960,000
Percent of total loans		5.80%
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	140,163,000	$ 211,815,000
Percent of total loans		2.30%
January 1, 2020 As Reported Under Topic 326
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	9,285,744,000	$ 9,305,357,000
Allowance for credit losses - loans	(130,488,000)	(100,559,000)
Net loans	$ 9,155,256,000	$ 9,204,798,000
Percent of total loans	100.00%	100.00%
January 1, 2020 As Reported Under Topic 326 | Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,024,089,000	$ 980,021,000
Percent of total loans	11.00%	10.50%
January 1, 2020 As Reported Under Topic 326 | Commercial real estate - investment properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,007,537,000	$ 2,024,988,000
Percent of total loans	21.60%	21.80%
January 1, 2020 As Reported Under Topic 326 | Multifamily real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 400,206,000	$ 388,388,000
Percent of total loans	4.30%	4.20%
January 1, 2020 As Reported Under Topic 326 | Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 205,476,000	$ 210,668,000
Percent of total loans	2.20%	2.30%
January 1, 2020 As Reported Under Topic 326 | Multifamily construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 250,410,000	$ 233,610,000
Percent of total loans	2.70%	2.50%
January 1, 2020 As Reported Under Topic 326 | One- to four- family construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 534,956,000	$ 544,308,000
Percent of total loans	5.80%	5.80%
January 1, 2020 As Reported Under Topic 326 | Land and Land Development Type [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 232,506,000	$ 245,530,000
Percent of total loans	2.50%	2.60%
January 1, 2020 As Reported Under Topic 326 | Commercial business
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,357,817,000	$ 1,364,650,000
Percent of total loans	14.60%	14.70%
January 1, 2020 As Reported Under Topic 326 | Small business scored
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 807,539,000	$ 772,657,000
Percent of total loans	8.70%	8.30%
January 1, 2020 As Reported Under Topic 326 | Small Balance Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 591,783,000	$ 613,484,000
Percent of total loans	6.40%	6.60%
January 1, 2020 As Reported Under Topic 326 | Agricultural business, including secured by farmland
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 330,257,000	$ 337,271,000
Percent of total loans	3.60%	3.60%
January 1, 2020 As Reported Under Topic 326 | One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 881,387,000	$ 925,531,000
Percent of total loans	9.50%	9.90%
January 1, 2020 As Reported Under Topic 326 | Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 521,618,000	$ 519,336,000
Percent of total loans	5.60%	5.60%
January 1, 2020 As Reported Under Topic 326 | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 140,163,000	$ 144,915,000
Percent of total loans	1.50%	1.60%