Federal funds purchased and securities sold under agreements to repurchase - Repurchase Agreements - Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	$ 30,764	$ 27,272
U.S. Treasury
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	779	754
U.S. government-sponsored enterprises
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	17,275	12,853
Asset-backed securities
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	12,710	13,665
Overnight and continuous
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	30,764	27,272
Overnight and continuous | U.S. Treasury
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	779	754
Overnight and continuous | U.S. government-sponsored enterprises
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	17,275	12,853
Overnight and continuous | Asset-backed securities
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	12,710	13,665
Less than 90 days
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	0	0
Less than 90 days | U.S. Treasury
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	0	0
Less than 90 days | U.S. government-sponsored enterprises
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	0	0
Less than 90 days | Asset-backed securities
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	0	0
Greater than 90 days
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	0	0
Greater than 90 days | U.S. Treasury
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	0	0
Greater than 90 days | U.S. government-sponsored enterprises
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	0	0
Greater than 90 days | Asset-backed securities
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	$ 0	$ 0