Income taxes - Reconciliation of Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. statutory tax rate	35.00%	35.00%	35.00%
Increase (decrease) in taxes resulting from:
State taxes (net of federal income tax benefit)	2.40%	2.40%	2.50%
Domestic manufacturing deduction	(3.00%)	(3.30%)	(2.10%)
Uncertain tax positions, settlements and lapse of statutes of limitations	(0.90%)	(0.50%)	0.60%
Net deferred tax write offs and deferred tax rate adjustments	(6.30%)	(2.40%)	(3.60%)
Enactment of the Tax Cuts and Jobs Act	(70.90%)	0.00%	0.00%
Non-deductible transactions costs	1.20%	0.80%	1.00%
Non-deductible goodwill	0.00%	0.00%	0.70%
Net excess benefits on share-based payments	(0.40%)	(1.40%)	(0.00%)
Other, net	(1.30%)	0.60%	(0.50%)
Effective tax rate	(44.20%)	31.20%	33.60%