LOANS RECEIVABLE AND THE ALLOWANCE FOR CREDIT LOSSES (Loans by Type) (Details) - USD ($)	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 9,654,181,000		$ 9,870,982,000
Allowance for credit losses - loans	(148,009,000)	$ (156,054,000)	(167,279,000)	$ (156,352,000)	$ (130,488,000)	$ (100,559,000)
Net loans	$ 9,506,172,000		9,703,703,000
Percent of total loans	100.00%
Unearned loan fees in excess of unamortized costs	$ 25,900,000		25,600,000
Discount on acquired loans, net	12,500,000		16,100,000
Financing Receivable, Accrued Interest, before Allowance for Credit Loss	35,100,000		36.6
Financing Receivable, Troubled Debt Restructuring	6,700,000		7,900,000
Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			9,870,982,000
Allowance for credit losses - loans			(167,279,000)
Net loans			$ 9,703,703,000
Percent of total loans			100.00%
Commerical real estate - owner-occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,066,237,000		$ 1,076,467,000
Percent of total loans	11.00%
Commerical real estate - owner-occupied [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 1,076,467,000
Percent of total loans			10.90%
Commercial real estate - investment properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 1,950,211,000		$ 1,955,684,000
Percent of total loans	20.20%
Commercial real estate - investment properties | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 1,955,684,000
Percent of total loans			19.80%
Small Balance Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 621,102,000
Percent of total loans	6.40%
Small Balance Commercial Real Estate Loans [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 573,849,000
Percent of total loans			5.80%
Multifamily Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 504,445,000		$ 428,223,000
Allowance for credit losses - loans	$ (5,807,000)	(4,367,000)	(3,893,000)	(3,504,000)	(2,805,000)	(4,754,000)
Percent of total loans	5.20%
Multifamily Real Estate | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 428,223,000
Percent of total loans			4.40%
Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 182,868,000		$ 228,937,000
Percent of total loans	1.90%
Commercial construction | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 228,937,000
Percent of total loans			2.30%
Multifamily construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 295,661,000		$ 305,527,000
Percent of total loans	3.10%
Multifamily construction | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 305,527,000
Percent of total loans			3.10%
One- to four- family construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 603,895,000		$ 507,810,000
Percent of total loans	6.30%
One- to four- family construction | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 507,810,000
Percent of total loans			5.10%
Land and Land Development Type [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 290,404,000
Percent of total loans	3.00%
Land and Land Development Type [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 248,915,000
Percent of total loans			2.50%
Land and land development - residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 290,404,000		$ 248,915,000
Commercial Business
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,931,531,000		2,178,461,000
Allowance for credit losses - loans	$ (30,830,000)	(31,411,000)	(35,007,000)	(33,870,000)	(31,648,000)	(23,370,000)
Percent of total loans	20.00%
Commercial Business | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 2,178,461,000
Percent of total loans			22.10%
Small business scored
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 743,975,000		$ 743,451,000
Percent of total loans	7.70%
Small business scored | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 743,451,000
Percent of total loans			7.50%
Agricultural business, including secured by farmland
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 265,429,000		$ 299,949,000
Allowance for credit losses - loans	$ (3,256,000)	(4,617,000)	(4,914,000)	(4,517,000)	(4,513,000)	(4,120,000)
Percent of total loans	2.70%
Loans And LeasesReceivable Gross Carrying Amount, SBA Paycheck Protection Program			0
Agricultural business, including secured by farmland | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 299,949,000
Percent of total loans			3.00%
One- to four-family residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 637,701,000		$ 717,939,000
Allowance for credit losses - loans	$ (9,800,000)	$ (7,988,000)	(9,913,000)	$ (12,746,000)	$ (11,884,000)	$ (4,136,000)
Percent of total loans	6.60%
One- to four-family residential [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 717,939,000
Percent of total loans			7.30%
Consumer secured by one- to four-family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 458,915,000		$ 491,812,000
Percent of total loans	4.80%
Consumer secured by one- to four-family [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 491,812,000
Percent of total loans			5.00%
Consumer-other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 101,807,000		$ 113,958,000
Percent of total loans	1.10%
Consumer-other | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable			$ 113,958,000
Percent of total loans			1.20%