Accumulated other comprehensive loss - Reclassifications out of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	$ 1,388	$ 1,031	$ 4,165	$ (17,708)
Income tax effect	(356)	(265)	(1,067)	4,585
Total reclassifications, net of tax	1,032	766	3,098	(13,123)
Amortization of prior service credit, net
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	(116)	(106)	(348)	(354)
Amortization of actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	1,504	1,137	4,513	3,446
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)