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Employee benefit plans - Schedule of Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Pension benefits        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Interest cost $ 3,200 $ 3,380 $ 9,600 $ 10,140
Expected return on assets (4,028) (4,299) (12,084) (12,897)
Amortization of net actuarial gain (loss) 1,037 773 3,111 2,319
Net periodic benefit cost (credit) 209 (146) 627 (438)
Other postretirement benefits        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Service cost 126 131 378 405
Interest cost 459 489 1,377 1,467
Expected return on assets (1,329) (1,334) (3,987) (4,026)
Amortization of prior service credit (330) (330) (990) (989)
Amortization of net actuarial gain (loss) (72) (96) (216) (408)
Net periodic benefit credit, including amount capitalized (1,146) (1,140) (3,438) (3,551)
Less amount capitalized 25 23 72 76
Net periodic benefit cost (credit) $ (1,171) $ (1,163) $ (3,510) $ (3,627)