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Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 129,158	$ 37,638
Gross Unrealized Gains		3
Gross Unrealized Losses	(24)	(3)
Estimated Fair Value	129,134	37,638
Money Market Funds | Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,213	14,334
Estimated Fair Value	22,213	14,334
Money Market Funds | Restricted cash (money market funds)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50	5,330
Gross Unrealized Gains		3
Estimated Fair Value	50	5,333
U.S. Government Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	93,246	16,474
Gross Unrealized Losses	(23)	(3)
Estimated Fair Value	93,223	16,471
Cash and Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	35,861	14,334
Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	93,223	17,971
Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,649	1,500
Gross Unrealized Losses	(1)
Estimated Fair Value	$ 13,648	$ 1,500

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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