Income taxes - reconciliations between statutory federal income tax rate and effective income tax rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory rate	35.00%	35.00%	35.00%
Tax effect of international items	(25.70%)	(27.50%)	(28.40%)
Impacts of U.S. tax reform	37.90%	0.00%	0.00%
Excess tax benefits related to share-based compensation	(2.30%)	0.00%	0.00%
State taxes, net of federal benefit	0.10%	0.90%	(0.70%)
Uncertain tax contingencies	(1.80%)	(3.60%)	(1.90%)
Contingent consideration reversals	0.40%	(1.20%)	(0.70%)
Other, net	1.90%	(0.30%)	(0.10%)
Effective income tax rate, total	45.50%	3.30%	3.20%