LOANS RECEIVABLE AND THE ALLOWANCE FOR CREDIT LOSSES (Loans by Type) (Details) - USD ($)	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 10,163,917,000		$ 9,305,357,000	$ 8,835,368,000
Allowance for credit losses - loans	(167,965,000)	$ (156,352,000)	(100,559,000)	(97,801,000)	$ (98,254,000)	$ (96,485,000)
Net loans	9,995,952,000		$ 9,204,798,000
Percent of total loans			100.00%
Unearned loan fees in excess of unamortized costs	31,700,000		$ 438,000
Discount on acquired loans, net	17,900,000		25,000,000.0
Financing Receivable, Accrued Interest, before Allowance for Credit Loss	39.2		31.8
Financing Receivable, Troubled Debt Restructuring	10,400,000		8,000,000.0
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,049,877,000		$ 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	1,991,258,000		$ 2,309,221,000
Percent of total loans			24.80%
Multifamily real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	426,659,000		$ 473,152,000	399,814,000
Allowance for credit losses - loans	(3,256,000)	(3,504,000)	$ (4,754,000)	(4,283,000)	(4,344,000)	(3,818,000)
Percent of total loans			5.10%
Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	220,285,000		$ 210,668,000
Percent of total loans			2.30%
Multifamily construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	291,105,000		$ 233,610,000
Percent of total loans			2.50%
One- to four- family construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	518,085,000		$ 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	240,803,000		$ 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	2,343,619,000		$ 1,693,824,000	1,619,391,000
Allowance for credit losses - loans	(35,369,000)	(33,870,000)	$ (23,370,000)	(21,147,000)	(19,557,000)	(19,438,000)
Percent of total loans			18.20%
Small business scored
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	763,824,000
Agricultural business, including secured by farmland
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	326,169,000		$ 370,549,000	390,505,000
Allowance for credit losses - loans	(5,051,000)	(4,517,000)	$ (4,120,000)	(3,895,000)	(3,691,000)	(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	771,431,000		$ 945,622,000	947,475,000
Allowance for credit losses - loans	(12,868,000)	$ (12,746,000)	$ (4,136,000)	$ (4,569,000)	$ (4,701,000)	$ (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	504,523,000		$ 550,960,000
Percent of total loans			5.80%
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	118,308,000		$ 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	10,163,917,000		$ 9,305,357,000
Net loans	$ 9,995,952,000		$ 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	$ 1,049,877,000		$ 980,021,000
Percent of total loans	10.30%		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	$ 1,991,258,000		$ 2,024,988,000
Percent of total loans	19.60%		21.80%
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	$ 597,971,000		$ 613,484,000
Percent of total loans	5.90%		6.60%
January 1, 2020 As Reported Under Topic 326 | Multifamily real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 426,659,000		$ 388,388,000
Percent of total loans	4.20%		4.20%
January 1, 2020 As Reported Under Topic 326 | Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 220,285,000		$ 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	$ 291,105,000		$ 233,610,000
Percent of total loans	2.90%		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	$ 518,085,000		$ 544,308,000
Percent of total loans	5.10%		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	$ 240,803,000		$ 245,530,000
Percent of total loans	2.40%		2.60%
January 1, 2020 As Reported Under Topic 326 | Commercial business
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 2,343,619,000		$ 1,364,650,000
Percent of total loans	23.10%		14.70%
January 1, 2020 As Reported Under Topic 326 | Small business scored
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 763,824,000		$ 772,657,000
Percent of total loans	7.50%		8.30%
January 1, 2020 As Reported Under Topic 326 | Agricultural business, including secured by farmland
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 326,169,000		$ 337,271,000
Percent of total loans	3.20%		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	$ 771,431,000		$ 925,531,000
Percent of total loans	7.60%		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	$ 504,523,000		$ 519,336,000
Percent of total loans	4.90%		5.60%
January 1, 2020 As Reported Under Topic 326 | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 118,308,000		$ 144,915,000
Percent of total loans	1.10%		1.60%