LOANS - Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended		12 Months Ended
	Dec. 31, 2020	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Financing Receivable, Recorded Investment [Line Items]
Accrued interest receivable for LHFI totaled with no related ACL		$ 17,300			$ 17,300
Interest income not recorded due to loans in non-accrual status						$ 1,100
Non- Accrual	$ 0	0			0		$ 0
TDR Period Increase (Decrease), Allowance for Loan and Lease Losses		91		$ 29	112	76
Allowance for Loan and Lease Losses Write-offs, Net					0	0
TDRs for increased allowance for credit losses for which there was a payment default within twelve months					101	54
Total loans, other financial institutions		120,800			120,800
Loans sold to other financial institutions.		72,500			72,500
Loan retained in other financial institutions.		48,300			48,300
PPP Loans	239,700	87,100			87,100		239,700
Provision for Loan and Lease Losses		0	$ 0	$ 6,921	0	$ 21,628
Amortized cost, PCD loans		10,600			10,600
PCD loans, estimated ACL		1,000			1,000
Trouble debt restructuring
Financing Receivable, Recorded Investment [Line Items]
TDRs total	$ 27,500				25,900
Additional amount committed on TDR loans		$ 0			0
TDRs allowance for loan losses					$ 3,000		$ 4,100