Accumulated other comprehensive loss - Reclassifications out of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	$ 1,327	$ 1,085	$ 2,777	$ (18,739)
Income tax effect	(339)	(279)	(711)	4,850
Total reclassifications, net of tax	988	806	2,066	(13,889)
Amortization of prior service credit, net
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	(107)	(123)	(232)	(248)
Amortization of actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	1,434	1,208	3,009	2,309
Realized gain on available-for-sale investment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	$ 0	$ 0	$ 0	$ (20,800)