Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk - Fair Value of Derivatives (Details)	1 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($) item	Sep. 30, 2020 USD ($) item	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Loan pool hedge	$ 50,000,000.0		$ 50,000,000.0
Term of interest rate risk cash flow hedge	5 years
Derivative assets designated as hedging instruments, fair value		$ 2,988,000
Weighted average maturity			5 years 10 months 24 days
Designated as Hedging Instrument [Member]
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Derivative liabilities designated as hedging instruments, fair value		$ 7,610,000
Designated as Hedging Instrument [Member] | Interest Rate Products
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Derivative, number of transactions | item	2	2	2
Notional Amount	$ 75,774,000	$ 75,774,000	$ 75,774,000
Derivative assets designated as hedging instruments, classified in other assets		2,988,000
Derivated liabilities designated as hedging instruments, classified in other liabilities	3,150,000	7,610,000	3,150,000
Derivative liabilities designated as hedging instruments, fair value	3,150,000		3,150,000
Designated as Hedging Instrument [Member] | Interest Rate Products | OSBC Affiliates
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Notional Amount				$ 25,800,000
Not Designated as Hedging Instrument [Member]
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Derivative assets not designated as hedging instruments, fair value		8,952,000
Total Derivative liabilities not designated as hedging instruments, fair value		$ 7,621,000
Not Designated as Hedging Instrument [Member] | Interest Rate Products
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Investment securities pledged with financial institutions, dollars	$ 11,000,000.0		$ 11,000,000.0
Not Designated as Hedging Instrument [Member] | Interest Rate Products | Commercial Loan Customers
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Derivative, number of transactions | item	25	28	25
Notional Amount	$ 177,872,000	$ 191,341,000	$ 177,872,000
Interest rate derivatives, derivative assets not designated as hedging instruments, classified in other assets	2,771,000	7,518,000	2,771,000
Interest rate derivatives, derivative not designated as hedging instruments, classified in other liabilities	2,771,000	$ 7,518,000	$ 2,771,000
Weighted average maturity		5 years	5 years 10 months 24 days
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Cash collateral pledged with financial institutions, dollars	$ 114,000		$ 114,000
Number of correspondent financial institutions where investment securities are pledged | item		1	1
Number of financial institutions where investment securities are required to be pledged | item			2
Not Designated as Hedging Instrument [Member] | Interest Rate Lock Commitments and Forward Contracts
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Derivative, number of transactions | item	4	250	4
Notional Amount	$ 28,176,000	$ 92,154,000	$ 28,176,000
Interest rate derivatives, derivative assets not designated as hedging instruments, classified in other assets		$ 1,434,000
Interest rate derivatives, derivative not designated as hedging instruments, classified in other liabilities	$ 53,000		$ 53,000
Not Designated as Hedging Instrument [Member] | Interest Rate Lock Commitments and Forward Contracts | Commercial Loan Customers
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Derivative, number of transactions | item	87		87
Notional Amount	$ 23,667,000		$ 23,667,000
Interest rate derivatives, derivative assets not designated as hedging instruments, classified in other assets	250,000		250,000
Not Designated as Hedging Instrument [Member] | Other Contracts
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Derivative, number of transactions | item		4
Notional Amount		$ 26,683,000
Derivative assets designated as hedging instruments, classified in other assets	3,021,000		3,021,000
Derivated liabilities designated as hedging instruments, classified in other liabilities	$ 2,824,000		$ 2,824,000
Interest rate derivatives, derivative not designated as hedging instruments, classified in other liabilities		103,000
Interest Income [Member]
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Amount to be reclassified as a reduction to interest expense during the next twelve months		180,000
Interest Expense [Member]
Derivatives, Hedging Activities and Financial Instruments with Off-Balance Sheet Risk
Amount to be reclassified as a reduction to interest expense during the next twelve months		$ 163,000