Financial Instruments - Summary of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 140,811	$ 186,320
Unrealized (Loss)	(173)	(270)
Estimated Fair Value	140,638	186,050
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cost	15,160	12,430
Estimated Fair Value	15,160	12,430
U.S. treasury securities
Schedule Of Available For Sale Securities [Line Items]
Cost	13,949	34,505
Unrealized (Loss)	(13)	(68)
Estimated Fair Value	13,936	34,437
U.S. government agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	33,423	53,832
Unrealized (Loss)	(122)	(141)
Estimated Fair Value	33,301	53,691
Corporate notes and commercial paper
Schedule Of Available For Sale Securities [Line Items]
Cost	78,279	85,553
Unrealized (Loss)	(38)	(61)
Estimated Fair Value	78,241	85,492
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	59,877	47,931
Short-term Investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	80,321	115,318
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value		22,361
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.

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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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

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

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This item represents the cost of debt and equity 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 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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