Federal funds purchased and securities sold under agreements to repurchase - Repurchase Agreements - Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	$ 22,097	$ 16,647
U.S. Treasury
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	738	1,464
U.S. government and federal agency obligations
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	2,378
Government sponsored enterprises
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	9,975	12,976
Asset-backed securities
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	9,006	2,207
Overnight and continuous
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	22,097	16,647
Overnight and continuous | U.S. Treasury
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	738	1,464
Overnight and continuous | U.S. government and federal agency obligations
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	2,378
Overnight and continuous | Government sponsored enterprises
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	9,975	12,976
Overnight and continuous | Asset-backed securities
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	9,006	2,207
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 and federal agency obligations
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	0
Less than 90 days | 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 and federal agency obligations
Federal funds purchased and securities sold under agreements to repurchase
Repurchase agreements	0
Greater than 90 days | 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