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Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 119,057	$ 37,637
Gross Unrealized Gains	2	3
Gross Unrealized Losses	(6)	(2)
Estimated Fair Value	119,053	37,638
Restricted cash
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50	5,330
Gross Unrealized Gains		3
Estimated Fair Value	50	5,333
Money Market Funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		14,334
Estimated Fair Value		14,334
Money Market Funds | Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,090
Estimated Fair Value	29,090
U.S. Government Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	74,144	16,473
Gross Unrealized Gains	1
Gross Unrealized Losses	(6)	(2)
Estimated Fair Value	74,139	16,471
Cash and Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	30,089	14,334
Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	88,914	17,971
Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,774	1,500
Gross Unrealized Gains	1
Estimated Fair Value	14,775	$ 1,500
Agency Securities | Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	999
Estimated Fair Value	$ 999

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

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- Definition

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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