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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 |  |

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