Cash, Cash Equivalents, and Investments - Summary of Available-for-Sale Investments (Detail) $ in Thousands	Mar. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 757,843
Gross unrealized gains	1,253
Gross unrealized losses	(705)
Fair value	758,391
Treasury bills
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(23)
Treasury bills | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	189,599
Gross unrealized gains	211
Gross unrealized losses	(2)
Fair value	189,808
Treasury bills | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	104,987
Gross unrealized gains	332
Gross unrealized losses	(21)
Fair value	105,298
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(505)
Corporate bonds | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	137,294
Gross unrealized gains	87
Gross unrealized losses	(153)
Fair value	137,228
Corporate bonds | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	152,668
Gross unrealized gains	329
Gross unrealized losses	(352)
Fair value	152,645
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(139)
Asset-backed securities | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	98,902
Gross unrealized gains	135
Gross unrealized losses	(127)
Fair value	98,910
Asset-backed securities | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	41,342
Gross unrealized gains	130
Gross unrealized losses	(12)
Fair value	41,460
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(9)
Municipal bonds | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	4,492
Gross unrealized gains	0
Gross unrealized losses	(1)
Fair value	4,491
Municipal bonds | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	855
Gross unrealized gains	0
Gross unrealized losses	(8)
Fair value	847
Sovereign bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(12)
Sovereign bonds | Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	10,898
Gross unrealized gains	14
Gross unrealized losses	(12)
Fair value	10,900
Sovereign bonds | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	10,876
Gross unrealized gains	15
Gross unrealized losses	0
Fair value	10,891
Agency bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	(17)
Agency bonds | Long-Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	5,930
Gross unrealized gains	0
Gross unrealized losses	(17)
Fair value	$ 5,913