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Derivatives and hedging activities - Fair Values of Derivative Instruments on the Balance Sheet (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($)
Derivatives, Fair Value
Asset Derivatives		$ 7,235	$ 661
Liability Derivatives		6,085	138
Interest Receivable		6,655	7,115
Interest Payable		1,461	1,195
Interest Rate Floor
Derivatives, Fair Value
Asset Derivatives		1,170	553
Interest Rate Swaps
Derivatives, Fair Value
Asset Derivatives		6,065	108
Derivatives designated as hedging instruments
Derivatives, Fair Value
Asset Derivatives		1,170	553
Derivatives designated as hedging instruments \| Interest Rate Floor
Derivatives, Fair Value
Notional amount		25
Asset Derivatives			553
Derivatives designated as hedging instruments \| Interest Rate Floor \| Other Assets.
Derivatives, Fair Value
Asset Derivatives		1,170
Derivatives designated as hedging instruments \| Interest Rate Swaps \| Other Assets.
Derivatives, Fair Value
Asset Derivatives		6,065	108
Derivatives designated as hedging instruments \| Interest Rate Swaps \| Other Liabilities.
Derivatives, Fair Value
Liability Derivatives		6,085	138
Derivatives not designated as hedging instruments
Derivatives, Fair Value
Asset Derivatives		6,065	108
Liability Derivatives		$ 6,085	138
Derivatives not designated as hedging instruments \| Interest Rate Swaps
Derivatives, Fair Value
Number of instruments held \| security		22
Notional amount		$ 126,574
Interest Receivable			13
Interest Payable			$ 13
Forecast \| Cash Flow Hedge \| Interest income
Derivatives, Fair Value
Amount of gain or (loss) reclassified as a reduction to interest income	$ 64