Employee benefits - Changes in level 3 assets measured at fair value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation
Fair value of plan assets at beginning of year	$ 619,885
Fair value of plan assets at end of year	652,426	$ 619,885
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation
Fair value of plan assets at beginning of year	3,581	4,566
Purchases, sales, issuance and settlements (net)	346	(985)
Fair value of plan assets at end of year	$ 3,927	$ 3,581