Accumulated other comprehensive loss - Reclassifications out of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	$ 1,551	$ 1,431	$ 4,653	$ 4,979
Income tax effect	(390)	(358)	(1,170)	(1,246)
Total reclassifications, net of tax	1,161	1,073	3,483	3,733
Amortization of prior service credit, net
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	(120)	(120)	(360)	(360)
Amortization of actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	1,671	1,551	5,013	4,653
Pension payment timing related charges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, before tax	$ 0	$ 0	$ 0	$ 686