Cash, Cash Equivalents and Investments - Amortized Cost to Fair Value (Detail) $ in Thousands	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 667,243
Gross Unrealized Gains	2,416
Gross Unrealized Losses	(65)
Fair Value, Total	669,594
Treasury Bills [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	309,248
Treasury Bills [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	92,782
Gross Unrealized Gains	137
Gross Unrealized Losses	(5)
Fair Value, Total	92,914
Treasury Bills [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	215,339
Gross Unrealized Gains	1,005
Gross Unrealized Losses	(10)
Fair Value, Total	216,334
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	113,083
Asset-Backed Securities [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	66,474
Gross Unrealized Gains	207
Gross Unrealized Losses	(1)
Fair Value, Total	66,680
Asset-Backed Securities [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	46,269
Gross Unrealized Gains	150
Gross Unrealized Losses	(16)
Fair Value, Total	46,403
Corporate Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	212,098
Corporate Bonds [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	65,440
Gross Unrealized Gains	188
Gross Unrealized Losses	(4)
Fair Value, Total	65,624
Corporate Bonds [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	145,806
Gross Unrealized Gains	674
Gross Unrealized Losses	(6)
Fair Value, Total	146,474
Sovereign Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	22,299
Sovereign Bonds [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,286
Gross Unrealized Gains	8
Gross Unrealized Losses	0
Fair Value, Total	6,294
Sovereign Bonds [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,971
Gross Unrealized Gains	40
Gross Unrealized Losses	(6)
Fair Value, Total	16,005
Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	6,952
Municipal Bonds [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,628
Gross Unrealized Gains	3
Gross Unrealized Losses	(1)
Fair Value, Total	4,630
Municipal Bonds [Member] | Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,318
Gross Unrealized Gains	4
Gross Unrealized Losses	0
Fair Value, Total	2,322
Agency Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	5,914
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	(16)
Fair Value, Total	$ 5,914