Cash, Cash Equivalents and Investments - Amortized Cost to Fair Value (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 691,319
Gross Unrealized Gains	323
Gross Unrealized Losses	(2,255)
Fair Value, Total	689,387
Treasury Bills [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	231,237
Treasury Bills [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	198,512
Gross Unrealized Gains	38
Gross Unrealized Losses	(73)
Fair Value, Total	198,477
Treasury Bills [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,722
Gross Unrealized Gains	51
Gross Unrealized Losses	(13)
Fair Value, Total	32,760
Corporate Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	300,437
Corporate Bonds [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	138,219
Gross Unrealized Gains	27
Gross Unrealized Losses	(375)
Fair Value, Total	137,871
Corporate Bonds [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	163,847
Gross Unrealized Gains	107
Gross Unrealized Losses	(1,388)
Fair Value, Total	162,566
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	132,038
Asset-Backed Securities [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	78,628
Gross Unrealized Gains	15
Gross Unrealized Losses	(236)
Fair Value, Total	78,407
Asset-Backed Securities [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,693
Gross Unrealized Gains	55
Gross Unrealized Losses	(117)
Fair Value, Total	53,631
Sovereign Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	14,417
Sovereign Bonds [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,135
Gross Unrealized Gains	0
Gross Unrealized Losses	(34)
Fair Value, Total	8,101
Sovereign Bonds [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,286
Gross Unrealized Gains	30
Gross Unrealized Losses	0
Fair Value, Total	6,316
Agency Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	5,921
Agency Bonds [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,930
Gross Unrealized Gains	0
Gross Unrealized Losses	(9)
Fair Value, Total	5,921
Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	5,337
Municipal Bonds [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,492
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value, Total	4,492
Municipal Bonds [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	855
Gross Unrealized Gains	0
Gross Unrealized Losses	(10)
Fair Value, Total	$ 845