Shareholders' equity - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Pension payment timing related charges	$ 7,498	$ 0	$ 0
Income before income taxes	508,707	310,716	449,291
Income tax effect	(107,367)	137,246	(140,171)
Total reclassifications, net of tax	9,439	37,212	4,940
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax	9,439	5,445	4,940
Retirement Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax	9,439	5,445	4,940
Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service cost	(403)	63	96
Amortization of actuarial loss	5,544	8,774	7,972
Pension payment timing related charges	7,498	0	0
Income before income taxes	12,639	8,837	8,068
Income tax effect	$ (3,200)	$ (3,392)	$ (3,128)