Available-for-Sale Securities and Fair Value Measurements - Available-for-Sale Securities (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Available-for-Sale Securities
Amortized Cost	$ 50,549	$ 122,727
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(18)
Estimated Fair Value	$ 50,546	122,709
Maturity period for marketable securities
Maximum contractual maturity period	1 year
Average maturity	1 month
U.S. government securities
Available-for-Sale Securities
Amortized Cost		9,943
Gross Unrealized Gains		0
Gross Unrealized Losses		(1)
Estimated Fair Value		9,942
U.S. government agencies
Available-for-Sale Securities
Amortized Cost		9,987
Gross Unrealized Gains		0
Gross Unrealized Losses		(2)
Estimated Fair Value		9,985
U.S. corporate notes
Available-for-Sale Securities
Amortized Cost	$ 3,876	10,881
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(15)
Estimated Fair Value	3,873	10,866
U.S. commercial papers
Available-for-Sale Securities
Amortized Cost	7,988	29,945
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	7,988	29,945
Money market funds
Available-for-Sale Securities
Amortized Cost	38,685	61,971
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 38,685	$ 61,971

Represents the average duration period for marketable debt securities.

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Represents the maximum maturity period for marketable debt securities.

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No definition available.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 10
-Section 50
-Paragraph 2
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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