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Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 135,733	$ 37,638
Gross Unrealized Gains	5	3
Gross Unrealized Losses	(1)	(3)
Estimated Fair Value	135,737	37,638
Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,500
Estimated Fair Value		1,500
Money Market Funds | Money Market Funds (Cash Equivalents)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,949	14,334
Estimated Fair Value	45,949	14,334
Money Market Funds | Restricted cash (money market funds)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	175	5,330
Gross Unrealized Gains		3
Estimated Fair Value	175	5,333
U.S. Government Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	89,609	16,474
Gross Unrealized Gains	5
Gross Unrealized Losses	(1)	(3)
Estimated Fair Value	89,613	16,471
Cash and Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	45,949	14,334
Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	$ 89,613	$ 17,971

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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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