Shareholders' equity - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 310,716	$ 449,291	$ 345,666
Income tax effect	137,246	(140,171)	(116,060)
Total reclassifications, net of tax	37,212	4,940	19,910
Retirement Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax	5,445	4,940	19,910
Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service cost	63	96	1,176
Amortization of actuarial loss	8,774	7,972	31,357
Income before income taxes	8,837	8,068	32,533
Income tax effect	$ (3,392)	$ (3,128)	$ (12,623)