Shareholders' equity - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pension payment timing related charges	$ 0	$ 686	$ 7,498
Income before income taxes	637,056	375,657	508,707
Income tax effect	(154,293)	(89,422)	(107,367)
Total reclassifications, net of tax	4,640	4,834	9,439
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax	4,640	4,834	9,439
Retirement Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax	4,640	4,834	9,439
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)	(403)
Amortization of actuarial loss	6,690	6,246	5,544
Pension payment timing related charges	0	686	7,498
Income before income taxes	6,209	6,451	12,639
Income tax effect	$ (1,569)	$ (1,617)	$ (3,200)