Accumulated other comprehensive loss - Reclassifications out of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	$ 2,236	$ 1,551	$ 4,815	$ 4,653
Income tax effect	(579)	(390)	(1,244)	(1,170)
Total reclassifications, net of tax	1,657	1,161	3,571	3,483
Amortization of prior service credit, net
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	(120)	(120)	(361)	(360)
Amortization of actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	1,410	1,671	4,230	5,013
Pension payment timing related charge
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	$ 946	$ 0	$ 946	$ 0