Fair value estimates - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Recurring Basis - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	$ 256,774	$ 259,410
U.S. Treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	20,080	7,438
U.S. Government-sponsored enterprises state and municipals
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	80,744	80,913
State and Municipals, Taxable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	15,295	15,225
State and Municipals, Tax-exempt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	104,869	112,600
Mortgage-backed Securities, U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	17,578	21,042
Mortgage-backed Securities, U.S. Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	18,208	22,192
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	20,080	7,438
Level 1 | U.S. Treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	20,080	7,438
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	236,694	251,972
Level 2 | U.S. Government-sponsored enterprises state and municipals
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	80,744	80,913
Level 2 | State and Municipals, Taxable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	15,295	15,225
Level 2 | State and Municipals, Tax-exempt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	104,869	112,600
Level 2 | Mortgage-backed Securities, U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	17,578	21,042
Level 2 | Mortgage-backed Securities, U.S. Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	$ 18,208	$ 22,192