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Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 143,673	$ 37,638
Gross Unrealized Gains		3
Gross Unrealized Losses	(4)	(3)
Estimated Fair Value	143,669	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	126,526	14,334
Estimated Fair Value	126,526	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	16,972	16,474
Gross Unrealized Losses	(4)	(3)
Estimated Fair Value	16,968	16,471
Cash and Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	126,526	14,334
Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	$ 16,968	$ 17,971

X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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-Section 50
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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

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-Publisher FASB
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-Topic 320
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-Paragraph 2
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+ References

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