Fair value of financial instruments - Loans Held for Sale at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	$ 821,529		$ 899,173
Commercial and industrial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	124,122		215,403
Residential real estate | 1-to-4 family mortgage
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	697,407		$ 683,770
Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	821,529	$ 899,173
Recurring Basis | Commercial and industrial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	124,122	215,403
Recurring Basis | Residential real estate | 1-to-4 family mortgage
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	$ 697,407	$ 683,770