Fair value estimates - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	$ 264,644	$ 259,410
U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	19,895	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	82,472	80,913
State and Municipals, Taxable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	15,267	15,225
State and Municipals, Tax-exempt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	107,537	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	19,332	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	20,141	22,192
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	19,895	7,438
Level 1 | U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	19,895	7,438
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	244,749	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	82,472	80,913
Level 2 | State and Municipals, Taxable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	15,267	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	107,537	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	19,332	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	$ 20,141	$ 22,192