Derivatives and hedging activities - Fair Values of Derivative Instruments on the Balance Sheet (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) security	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)
Derivatives, Fair Value
Asset Derivatives	$ 18,191		$ 5,672
Liability Derivatives	16,935		4,680
Interest Receivable	8,595		6,981
Interest Payable	1,289		1,277
Net Amounts of Assets presented in the Balance Sheet	16,935		4,680
Interest income
Derivatives, Fair Value
Amount of gain or (loss) reclassified as a increase to interest income (expense)	149
Interest Rate Floor
Derivatives, Fair Value
Asset Derivatives	1,845		944
Interest Rate Swaps
Derivatives, Fair Value
Asset Derivatives	16,346		4,728
Credit-risk contract
Derivatives, Fair Value
Posted collateral	16,450		4,140
Net Amounts of Assets presented in the Balance Sheet	4		48
Derivatives designated as hedging instruments
Derivatives, Fair Value
Asset Derivatives	1,845		944
Liability Derivatives	592
Derivatives designated as hedging instruments | Interest Rate Floor
Derivatives, Fair Value
Notional amount	25,000
Derivatives designated as hedging instruments | Interest Rate Floor | Other Assets
Derivatives, Fair Value
Asset Derivatives	1,845		944
Derivatives not designated as hedging instruments
Derivatives, Fair Value
Asset Derivatives	16,346		4,728
Liability Derivatives	$ 16,343		4,680
Derivatives not designated as hedging instruments | Interest Rate Swaps
Derivatives, Fair Value
Number of instruments held | security	64
Notional amount	$ 355,594			$ 126,574
Interest Receivable			211
Interest Payable			211
Derivatives not designated as hedging instruments | Interest Rate Swaps | Other Assets
Derivatives, Fair Value
Asset Derivatives	16,346		4,728
Derivatives not designated as hedging instruments | Interest Rate Swaps | Other Liabilities.
Derivatives, Fair Value
Liability Derivatives	16,343		$ 4,680
Cash Flow Hedge | Derivatives designated as hedging instruments
Derivatives, Fair Value
Notional amount	50,000
Cash Flow Hedge | Derivatives designated as hedging instruments | Other Liabilities.
Derivatives, Fair Value
Liability Derivatives	$ 592
Forecast | Cash Flow Hedge | Interest expense
Derivatives, Fair Value
Amount of gain or (loss) reclassified as a increase to interest income (expense)		$ 134
Forecast | Cash Flow Hedge | Interest income
Derivatives, Fair Value
Amount of gain or (loss) reclassified as a increase to interest income (expense)		$ 531