Shareholders' equity - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pension payment timing related charges	$ 946	$ 0	$ 686
Income before income taxes	613,678	637,056	375,657
Income tax effect	(135,481)	(154,293)	(89,422)
Total reclassifications, net of tax	(4,573)	(4,640)	(4,834)
Retirement Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax	(4,573)	(4,640)	(4,834)
Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service cost	(481)	(481)	(481)
Amortization of actuarial loss	5,698	6,690	6,246
Pension payment timing related charges	946	0	686
Income before income taxes	6,163	6,209	6,451
Income tax effect	(1,590)	(1,569)	(1,617)
Total reclassifications, net of tax	$ 4,573	$ 4,640	$ 4,834