Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Money market instruments	$ 10,883	$ 3,557
Financial assets at fair value	758,391
Corporate bonds
Assets:
Financial assets at fair value	289,873
Treasury bills
Assets:
Financial assets at fair value	295,106
Asset-backed securities
Assets:
Financial assets at fair value	140,370
Sovereign bonds
Assets:
Financial assets at fair value	21,791
Municipal bonds
Assets:
Financial assets at fair value	5,338
Agency bonds
Assets:
Financial assets at fair value	5,913
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Money market instruments	10,883
Economic hedge forward contracts	0
Liabilities:
Economic hedge forward contracts	0
Contingent consideration liabilities	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Treasury bills
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed securities
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Sovereign bonds
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Agency bonds
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Money market instruments	0
Economic hedge forward contracts	34
Liabilities:
Economic hedge forward contracts	31
Contingent consideration liabilities	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate bonds
Assets:
Financial assets at fair value	289,873
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Treasury bills
Assets:
Financial assets at fair value	295,106
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Assets:
Financial assets at fair value	140,370
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Sovereign bonds
Assets:
Financial assets at fair value	21,791
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds
Assets:
Financial assets at fair value	5,338
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Agency bonds
Assets:
Financial assets at fair value	5,913
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3)
Assets:
Money market instruments	0
Economic hedge forward contracts	0
Liabilities:
Economic hedge forward contracts	0
Contingent consideration liabilities	1,569
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) | Corporate bonds
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) | Treasury bills
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) | Asset-backed securities
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) | Sovereign bonds
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) | Municipal bonds
Assets:
Financial assets at fair value	0
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) | Agency bonds
Assets:
Financial assets at fair value	$ 0