Cash, Cash Equivalents and Investments - Fair value and Amortized cost (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash and money market funds
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	$ 166,144	$ 114,504
Estimated Fair Value	166,144	114,504
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	34,846
Gross Unrealized Gains	48
Estimated Fair Value	34,894
Total
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	200,990	114,504
Gross Unrealized Gains	48
Estimated Fair Value	201,038	114,504
Less amounts classified as cash and cash equivalents
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	166,144	114,504
Estimated Fair Value	166,144	$ 114,504
Total available-for-sale investments
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	34,846
Gross Unrealized Gains	48
Estimated Fair Value	$ 34,894

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).

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=118939692&loc=d3e27161-111563

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- Definition

Amortized cost of 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://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

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- Definition

Amount of 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://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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- References

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