Short-term borrowings (Details) - Repurchase agreement - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets Sold Under Agreements To Repurchase [Line Items]
Total repurchase agreements	$ 79,381	$ 77,926
U.S. Treasury and U.S. government sponsored entities
Assets Sold Under Agreements To Repurchase [Line Items]
Total repurchase agreements	4,941	5,051
State and political subdivisions
Assets Sold Under Agreements To Repurchase [Line Items]
Total repurchase agreements	24,855	21,731
Mortgage-backed securities - residential
Assets Sold Under Agreements To Repurchase [Line Items]
Total repurchase agreements	44,829	46,133
Collateralized mortgage obligations - residential
Assets Sold Under Agreements To Repurchase [Line Items]
Total repurchase agreements	$ 4,756	$ 5,011