Commitments and contingencies Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Commitments to Extend Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability	$ 2,502,731	$ 2,443,478
Standby Letters of Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability	15,994	15,534
Deferred standby letter of credit fees [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability	$ 114	$ 102