Fair value measurement - financial assets and liabilities carried at fair value measured on recurring basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	$ 9,045	$ 7,660
Derivative assets	1,221	1,157
Derivative liabilities	1,426	2,257
Contingent consideration liabilities	272,136	7,102
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	9,045	7,660
Derivative assets	0	0
Derivative liabilities	0	0
Contingent consideration liabilities	0	0
Significant Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	0	0
Derivative assets	1,221	1,157
Derivative liabilities	1,426	2,257
Contingent consideration liabilities	0	0
Significant Unobservable Inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	0	0
Derivative assets	0	0
Derivative liabilities	0	0
Contingent consideration liabilities	$ 272,136	$ 7,102