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Employee benefit plans - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Retirement Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 3,456 $ 2,878 $ 3,266
Interest cost 19,097 18,101 10,562
Expected return on plan assets (11,133) (9,451) (10,302)
Amortization of prior service cost / (credit) 864 926 913
Amortization of net actuarial gain (loss) 4,472 4,300 6,240
Effect of settlement loss 530 1,010 479
Effect of curtailment loss 0 0 43
Net periodic benefit cost/(income) 17,286 17,764 11,201
Retiree Health and Life Insurance Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 178 230 320
Interest cost 919 507 258
Expected return on plan assets (392) (313) (439)
Amortization of prior service cost / (credit) 385 0 0
Amortization of net actuarial gain (loss) (899) (768) (681)
Net periodic benefit cost/(income) $ 191 $ (344) $ (542)