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LOANS AND ALLOWANCE FOR CREDIT LOSSES - Financing Receivable, Allowance for Credit Loss (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
	Jan. 01, 2021	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Provision for Loan and Lease Losses		$ 300	$ 3,871
Allowance for credit losses
Provision for Loan and Lease Losses	$ 67,338	67,702		$ 55,743
Provision (benefit) charged to expense		300
Losses charged off		(655)
Recoveries		719
CECL adoption \| Allowance for credit losses
Provision for Loan and Lease Losses	11,595
Loans Receivable
Provision for Loan and Lease Losses			43,928		$ 40,294
Provision (benefit) charged to expense			3,871
Losses charged off			(1,145)
Recoveries			908
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment				713
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment				54,350
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses				680
Financing Receivable, Individually Evaluated for Impairment				9,048
Financing Receivable, Collectively Evaluated for Impairment				5,117,838
All Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Not Accounted for Using Income Recognition Model				98,643
Loans Receivable \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	8,690	8,017
Provision (benefit) charged to expense		(673)
Loans Receivable \| CECL adoption \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	8,690
Commercial Real Estate
Provision for Loan and Lease Losses			25,923		24,334
Provision (benefit) charged to expense			1,549
Recoveries			40
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment				445
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment				32,937
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses				325
Financing Receivable, Individually Evaluated for Impairment				3,438
Financing Receivable, Collectively Evaluated for Impairment				1,550,239
All Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Not Accounted for Using Income Recognition Model				24,613
Commercial Real Estate \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	354	457
Provision (benefit) charged to expense		103
Commercial Real Estate \| Allowance for credit losses
Provision for Loan and Lease Losses	31,176	31,500		33,707
Provision (benefit) charged to expense		300
Recoveries		24
Commercial Real Estate \| CECL adoption \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	354
Commercial Real Estate \| CECL adoption \| Allowance for credit losses
Provision for Loan and Lease Losses	(2,531)
Commercial construction
Provision for Loan and Lease Losses			2,221		3,076
Provision (benefit) charged to expense			(867)
Losses charged off			(1)
Recoveries			13
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment				3,378
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses				143
Financing Receivable, Collectively Evaluated for Impairment				1,266,847
All Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Not Accounted for Using Income Recognition Model				2,551
Commercial construction \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	910	510
Provision (benefit) charged to expense		(400)
Commercial construction \| Allowance for credit losses
Provision for Loan and Lease Losses	2,356	2,366		3,521
Recoveries		10
Commercial construction \| CECL adoption \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	910
Commercial construction \| CECL adoption \| Allowance for credit losses
Provision for Loan and Lease Losses	(1,165)
One- to four-family residential construction
Provision for Loan and Lease Losses			4,739		4,339
Provision (benefit) charged to expense			394
Losses charged off			(29)
Recoveries			35
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment				90
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment				4,382
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses				64
Financing Receivable, Individually Evaluated for Impairment				3,546
Financing Receivable, Collectively Evaluated for Impairment				655,146
All Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Not Accounted for Using Income Recognition Model				57,113
One- to four-family residential construction \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	917	957
Provision (benefit) charged to expense		40
One- to four-family residential construction \| Allowance for credit losses
Provision for Loan and Lease Losses	9,069	9,101		4,536
Losses charged off		(6)
Recoveries		38
One- to four-family residential construction \| CECL adoption \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	917
One- to four-family residential construction \| CECL adoption \| Allowance for credit losses
Provision for Loan and Lease Losses	4,533
Other residential
Provision for Loan and Lease Losses			6,146		5,153
Provision (benefit) charged to expense			879
Recoveries			114
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment				9,282
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses				93
Financing Receivable, Collectively Evaluated for Impairment				1,021,145
All Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Not Accounted for Using Income Recognition Model				6,150
Other residential \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	5,227	4,815
Provision (benefit) charged to expense		(412)
Other residential \| Allowance for credit losses
Provision for Loan and Lease Losses	15,207	15,299		9,375
Recoveries		92
Other residential \| CECL adoption \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	5,227
Other residential \| CECL adoption \| Allowance for credit losses
Provision for Loan and Lease Losses	5,832
Commercial business
Provision for Loan and Lease Losses			1,579		1,355
Provision (benefit) charged to expense			169
Losses charged off			(9)
Recoveries			64
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment				14
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment				2,331
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses				45
Financing Receivable, Individually Evaluated for Impairment				167
Financing Receivable, Collectively Evaluated for Impairment				384,734
All Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Not Accounted for Using Income Recognition Model				2,549
Commercial business \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	935	956
Provision (benefit) charged to expense		21
Commercial business \| Allowance for credit losses
Provision for Loan and Lease Losses	3,889	3,936		2,390
Recoveries		47
Commercial business \| CECL adoption \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	935
Commercial business \| CECL adoption \| Allowance for credit losses
Provision for Loan and Lease Losses	1,499
Consumer
Provision for Loan and Lease Losses			3,320		$ 2,037
Provision (benefit) charged to expense			1,747
Losses charged off			(1,106)
Recoveries			$ 642
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment				164
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment				2,040
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses				10
Financing Receivable, Individually Evaluated for Impairment				1,897
Financing Receivable, Collectively Evaluated for Impairment				239,727
All Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Not Accounted for Using Income Recognition Model				5,667
Consumer \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	347	322
Provision (benefit) charged to expense		(25)
Consumer \| Allowance for credit losses
Provision for Loan and Lease Losses	5,641	5,500		$ 2,214
Losses charged off		(649)
Recoveries		$ 508
Consumer \| CECL adoption \| Allowance for unfunded commitment
Provision for Loan and Lease Losses	347
Consumer \| CECL adoption \| Allowance for credit losses
Provision for Loan and Lease Losses	$ 3,427