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Employee benefit plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Components of net periodic pension cost:        
Salaries and employee benefits expense $ 7,829 $ 7,831 $ 21,649 $ 22,735
Pension Benefits        
Components of net periodic pension cost:        
Salaries and employee benefits expense 343 308 991 968
Interest cost 105 164 315 381
Expected return on plan assets (322) (371) (966) (865)
Amortization of unrecognized net gain 76 65 227 152
Net periodic benefit $ (141) $ (142) $ (424) $ (332)