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Loans and Allowance for Credit Losses - Troubled Debt Restructuring and CARES Act Narrative (Details)	9 Months Ended	10 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) borrowingRelationship loan loanProductionOffice	Dec. 31, 2020 USD ($) loan borrowingRelationship	Dec. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan	Oct. 01, 2020 USD ($)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans removed from TDR designation | loan			0
Loan modifications under CARES Act during period | loan		269
Loans modified under CARES Act during period, value		$ 402,900,000
Loan modifications under CARES Act during period, subsequently resumed payments | loanProductionOffice		222
Loans modified under CARES Act during period, subsequently resumed payments, value		$ 324,200,000
Loan modifications under CARES Act during period, subsequently fully paid | loanProductionOffice		18
Loan modifications under CARES Act during period, subsequently fully paid, value		$ 7,700,000
Loan modifications under CARES Act, additional relief, number of borrowing relationships | borrowingRelationship		21	21
Loan modifications under CARES Act, additional relief | loan		29	29
Loan modifications under CARES Act, additional relief, value		$ 71,000,000.0	$ 71,000,000.0
Total of TDR loans charged-off within previous 12 months, value			0	$ 0	$ 0
Provision for credit losses on loans	$ 5,450,000		4,594,000	900,000	$ 0
Allowance for Credit Losses	22,113,000	22,874,000	22,874,000	$ 16,677,000
Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Troubled Debt Restructuring [Line Items]
Allowance for Credit Losses	1,604,000
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13
Financing Receivable, Troubled Debt Restructuring [Line Items]
Allowance for Credit Losses						$ 1,604,000
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans removed from TDR designation | loan				1
Recorded investment of loans removed from TDR resignation				$ 3,000
Residential loans | TIC Loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans removed from TDR designation | loan					1
Residential loans | Home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans removed from TDR designation | loan					1
Provision for credit losses on loans	188,000			$ (65,000)	$ (116,000)
Allowance for Credit Losses	1,038,000	$ 738,000	$ 738,000
Residential loans | Home equity | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Troubled Debt Restructuring [Line Items]
Allowance for Credit Losses	$ (361,000)
Residential loans | Home equity | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13
Financing Receivable, Troubled Debt Restructuring [Line Items]
Allowance for Credit Losses						$ (361,000)
Residential loans | TIC and Home Equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Recorded investment of loans removed from TDR resignation					$ 247,000
Consumer loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDR loans charged-off within previous 12 months | loan			1
Total of TDR loans charged-off within previous 12 months, value			$ 7,000

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