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Employee benefit plans - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Retirement Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 3,304 $ 3,916 $ 3,969
Interest cost 10,562 24,186 51,297
Expected return on plan assets (10,302) (22,888) (50,733)
Amortization of prior service cost / (credit) 913 900 1,006
Amortization of net actuarial gain (loss) 6,240 16,503 28,833
Effect of settlement loss 479 550,706 854
Effect of curtailment loss 43 0 32
Net periodic benefit cost (income) 11,239 573,323 35,258
Retiree Health and Life Insurance Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 320 374 358
Interest cost 258 197 336
Expected return on plan assets (439) (444) (371)
Amortization of prior service cost / (credit) 0 0 (279)
Amortization of net actuarial gain (loss) (681) (744) (834)
Net periodic benefit cost (income) $ (542) $ (617) $ (790)