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Employee benefit plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Components of net periodic pension cost:        
Salaries and employee benefits expense $ 7,250 $ 7,048 $ 13,820 $ 14,904
Pension Benefits        
Components of net periodic pension cost:        
Salaries and employee benefits expense 342 353 648 660
Interest cost 105 163 210 217
Expected return on plan assets (322) (371) (644) (494)
Amortization of unrecognized net gain 76 65 152 87
Net periodic benefit $ (141) $ (143) $ (282) $ (190)