Employee benefit obligations - Net Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Change in Benefit Obligation
Projected benefit obligation at beginning of year	$ 29,435	$ 29,975
Interest cost	$ 850	$ 62
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Company service cost	$ 2,992	$ 2,725
Employee contributions	2,517	1,631
Prior service cost	(768)	484
Benefits paid	6,726	423
Actuarial loss	9,305	(5,865)
Projected benefit obligation at end of year	51,057	29,435
Change in Plan Assets
Fair value of plan assets at beginning of year	25,531	25,967
Actual return on plan assets	4,699	(4,937)
Employer contributions	3,776	2,446
Employee contributions	2,518	1,631
Benefits paid	6,726	424
Fair value of plan assets at end of year	43,250	25,531
Funded Status at End of year
Excess of obligation over assets	7,807	3,904
Change in Accrued Benefit Liability
Accrued benefit liability at beginning of year	(3,904)	(4,008)
Company contributions made during year	3,776	2,446
Net periodic benefit cost for year	(3,058)	(2,078)
Net decrease (increase) in accumulated other comprehensive loss	(4,621)	(264)
Accrued benefit liability at end of year	(7,807)	(3,904)
Non-current plan assets	43,250	25,531
Non-current liability	51,057	29,435
Accrued benefit liability at end of year	(7,807)	(3,904)
Projected Benefit Payments
Projected year 1	1,138	539
Projected year 2	1,102	744
Projected year 3	1,546	983
Projected year 4	1,577	1,065
Projected year 5	958	578
Projected years 6-10	$ 20,127	$ 16,940