Cash, Cash Equivalents, and Investments - Summary of Available-for-Sale Investments (Detail) $ in Thousands	Jun. 30, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 769,801
Gross unrealized gains	2,985
Gross unrealized losses	(267)
Fair value	772,519
Agency Bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(27)
Agency Bonds | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	7,780
Gross unrealized gains	3
Gross unrealized losses	0
Fair value	7,783
Agency Bonds | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	5,930
Gross unrealized gains	0
Gross unrealized losses	(27)
Fair value	5,903
Treasury Bills
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(20)
Treasury Bills | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	138,217
Gross unrealized gains	383
Gross unrealized losses	0
Fair value	138,600
Treasury Bills | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	167,653
Gross unrealized gains	1,257
Gross unrealized losses	(20)
Fair value	168,890
Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(176)
Corporate Bonds | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	101,722
Gross unrealized gains	182
Gross unrealized losses	(36)
Fair value	101,868
Corporate Bonds | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	176,726
Gross unrealized gains	537
Gross unrealized losses	(140)
Fair value	177,123
Asset-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(37)
Asset-Backed Securities | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	104,717
Gross unrealized gains	317
Gross unrealized losses	(36)
Fair value	104,998
Asset-Backed Securities | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	40,198
Gross unrealized gains	237
Gross unrealized losses	(1)
Fair value	40,434
Sovereign Bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(1)
Sovereign Bonds | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	5,979
Gross unrealized gains	12
Gross unrealized losses	(1)
Fair value	5,990
Sovereign Bonds | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	14,783
Gross unrealized gains	50
Gross unrealized losses	0
Fair value	14,833
Municipal Bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(6)
Municipal Bonds | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	5,241
Gross unrealized gains	7
Gross unrealized losses	0
Fair value	5,248
Municipal Bonds | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	855
Gross unrealized gains	0
Gross unrealized losses	(6)
Fair value	$ 849