Commitments and contingent liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Commitments to Extend Credit [Member]
Other Commitments [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Liability	$ 2,590,000	$ 2,620,000
Standby Letters of Credit [Member]
Other Commitments [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Liability	26,900	28,300
Commercial Letter of Credit and other off-balance sheet liabilities [Member]
Other Commitments [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Liability	$ 24	$ 0