Employee benefit plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,918	$ 3,304	$ 3,916
Interest cost	18,101	10,562	24,186
Expected return on plan assets	(9,451)	(10,302)	(22,888)
Amortization of prior service cost / (credit)	926	913	900
Amortization of net actuarial gain (loss)	4,300	6,240	16,503
Effect of settlement loss	1,010	479	550,706
Effect of curtailment loss	0	43	0
Net periodic benefit cost (income)	17,804	11,239	573,323
Retiree Health and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	230	320	374
Interest cost	507	258	197
Expected return on plan assets	(313)	(439)	(444)
Amortization of prior service cost / (credit)	0	0	0
Amortization of net actuarial gain (loss)	(768)	(681)	(744)
Net periodic benefit cost (income)	$ (344)	$ (542)	$ (617)