Available-for-Sale Securities and Fair Value Measurements - Available-for-Sale Securities (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Available-for-Sale Securities
Amortized Cost	$ 51,456	$ 122,727
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(22)	(18)
Estimated Fair Value	$ 51,434	122,709
Maturity period for marketable securities
Maximum contractual maturity period	1 year
Average maturity	2 months
U.S. government securities
Available-for-Sale Securities
Amortized Cost	$ 9,978	9,943
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(1)
Estimated Fair Value	9,975	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	11,884	10,881
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(19)	(15)
Estimated Fair Value	11,865	10,866
U.S. commercial papers
Available-for-Sale Securities
Amortized Cost	12,476	29,945
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	12,476	29,945
Money market funds
Available-for-Sale Securities
Amortized Cost	17,118	61,971
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 17,118	$ 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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- Definition

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

+ References

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

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- Definition

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62557-112803

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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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