Financial Instruments - Summary of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 133,238	$ 136,329
Unrealized Gain	67	4
Unrealized (Loss)	(2)	(117)
Estimated Fair Value	133,303	136,216
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	12,782	14,077
Estimated Fair Value	12,782	14,077
U.S. treasury securities
Schedule Of Available For Sale Securities [Line Items]
Cost	61,166	20,334
Unrealized Gain	37	4
Unrealized (Loss)		(4)
Estimated Fair Value	61,203	20,334
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	5,809	28,149
Unrealized Gain	1
Unrealized (Loss)		(77)
Estimated Fair Value	5,810	28,072
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	53,481	73,769
Unrealized Gain	29
Unrealized (Loss)	(2)	(36)
Estimated Fair Value	53,508	73,733
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	20,930	50,681
Short-term Investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	111,933	85,095
Restricted Cash
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 440	$ 440

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

+ References

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+ References

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