Fair value estimates - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	$ 249,913	$ 284,935
U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value		9,999
U.S. Government-sponsored enterprises state and municipals
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	67,656	69,060
State and Municipals, Taxable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	15,754	16,545
State and Municipals, Tax-exempt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	118,297	124,924
Mortgage-backed Securities, U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	23,381	31,568
Mortgage-backed Securities, U.S. Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	24,825	32,839
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value		9,999
Level 1 | U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value		9,999
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	249,913	274,936
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	67,656	69,060
Level 2 | State and Municipals, Taxable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	15,754	16,545
Level 2 | State and Municipals, Tax-exempt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Assets measured at fair value	118,297	124,924
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	23,381	31,568
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	$ 24,825	$ 32,839