Income taxes - Reconciliation of U.S. Federal Statutory Tax Rate to Actual Consolidated Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Statutory tax rate	$ 79,961	$ 79,491	$ 110,094
State income taxes, net of federal tax benefit	7,767	7,534	4,780
Valuation allowance	3,174	(14,902)	(3,333)
Tax examinations including change in reserve for uncertain tax positions	(1,639)	(3,076)	4,895
Adjustments to prior year deferred taxes	(499)	(1,899)	(1,415)
Foreign earnings taxed at other than U.S. rates	5,083	8,224	(16,233)
Disposition of business	0	0	537
Effect of tax rate changes enacted during the year	531	(6,218)	(22,183)
Deduction related to qualified production activities	0	341	(5,384)
Transition tax	0	3,647	76,933
Tax credits	(13,310)	(10,083)	(1,197)
Global intangible low-taxed income (GILTI)	12,340	12,878	0
Foreign-derived intangible income	(1,225)	(1,174)	0
Other, net	1,086	245	(905)
Total taxes	$ 93,269	$ 75,008	$ 146,589
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory tax rate	21.00%	21.00%	35.00%
State income taxes, net of federal tax benefit	2.00%	2.00%	1.50%
Valuation allowance	0.80%	(3.90%)	(1.10%)
Tax examinations including change in reserve for uncertain tax positions	(0.40%)	(0.80%)	1.60%
Adjustments to prior year deferred taxes	(0.10%)	(0.50%)	(0.40%)
Foreign earnings taxed at other than U.S. rates	1.30%	2.20%	(5.20%)
Disposition of business	0.00%	0.00%	0.20%
Effect of tax rate changes enacted during the year	0.10%	(1.60%)	(7.10%)
Deduction related to qualified production activities	0.00%	0.10%	(1.70%)
Transition tax	0.00%	1.00%	24.50%
Tax credit	(3.50%)	(2.70%)	(0.40%)
Global intangible low-tax income (GILTI)	3.20%	3.40%	0.00%
Foreign-derivative intangible income	(0.30%)	(0.30%)	0.00%
Other, net	0.30%	0.10%	(0.30%)
Total taxes	24.50%	19.80%	46.60%