Cash, Cash Equivalents, and Investments - Summary of Available-for-Sale Investments (Detail) $ in Thousands	Sep. 30, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 702,797
Gross unrealized gains	608
Gross unrealized losses	(1,460)
Fair value	701,945
Agency Bonds \| Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,969
Gross unrealized gains	0
Gross unrealized losses	0
Fair value	2,969
Agency Bonds \| Long-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	5,930
Gross unrealized gains	10
Gross unrealized losses	0
Fair value	5,940
Treasury Bills
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses	(298)
Treasury Bills \| Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	193,631
Gross unrealized gains	0
Gross unrealized losses	(284)
Fair value	193,347
Treasury Bills \| Long-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	32,718
Gross unrealized gains	7
Gross unrealized losses	(14)
Fair value	32,711
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses	(683)
Corporate Bonds \| Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	138,383
Gross unrealized gains	56
Gross unrealized losses	(340)
Fair value	138,099
Corporate Bonds \| Long-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	175,881
Gross unrealized gains	469
Gross unrealized losses	(343)
Fair value	176,007
Asset-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses	(407)
Asset-Backed Securities \| Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	71,646
Gross unrealized gains	1
Gross unrealized losses	(209)
Fair value	71,438
Asset-Backed Securities \| Long-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	60,276
Gross unrealized gains	33
Gross unrealized losses	(198)
Fair value	60,111
Sovereign Bonds
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses	(53)
Sovereign Bonds \| Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	8,172
Gross unrealized gains	0
Gross unrealized losses	(53)
Fair value	8,119
Sovereign Bonds \| Long-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	6,286
Gross unrealized gains	32
Gross unrealized losses	0
Fair value	6,318
Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses	(19)
Municipal Bonds \| Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	6,050
Gross unrealized gains	0
Gross unrealized losses	(6)
Fair value	6,044
Municipal Bonds \| Long-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	855
Gross unrealized gains	0
Gross unrealized losses	(13)
Fair value	$ 842

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section S99
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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-Name Accounting Standards Codification
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-Section 50
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