Employee benefits - Changes in level 3 assets measured at fair value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation
Fair value of plan assets at beginning of year	$ 860,484
Fair value of plan assets at end of year	619,885	$ 860,484
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation
Fair value of plan assets at beginning of year	4,566	3,917
Purchases, sales, issuance, settlements, paydowns and maturities (net)	(985)	649
Fair value of plan assets at end of year	$ 3,581	$ 4,566