Employee benefit obligations - Net Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Change in Benefit Obligation
Projected benefit obligation at beginning of year	$ 22,753	$ 15,685
Interest cost	48	37
Company service cost	1,915	1,483
Employee contributions	1,167	870
Prior service cost	(923)	0
Benefits paid	2,976	1,612
Actuarial loss	2,039	3,066
Projected benefit obligation at end of year	29,975	22,753
Change in Plan Assets
Fair value of plan assets at beginning of year	18,082	12,356
Actual return on plan assets	1,992	1,938
Employer contributions	1,750	1,306
Employee contributions	1,167	870
Benefits paid	2,976	1,612
Fair value of plan assets at end of year	25,967	18,082
Funded Status at End of year
Excess of obligation over assets	4,008	4,671
Change in Accrued Benefit Liability
Accrued benefit liability at beginning of year	(4,671)	(3,329)
Company contributions made during year	1,750	1,306
Net periodic benefit cost for year	(1,310)	(1,909)
Net decrease (increase) in accumulated other comprehensive loss	223	(739)
Accrued benefit liability at end of year	(4,008)	(4,671)
Non-current plan assets	25,967	18,083
Non-current liability	29,975	22,754
Accrued benefit liability at end of year	(4,008)	(4,671)
Projected Benefit Payments
Projected year 1	567	1,804
Projected year 2	454	394
Projected year 3	819	400
Projected year 4	765	788
Projected year 5	452	405
Projected years 6-10	$ 13,816	$ 3,445