Fair Value Measurements - (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair value of the Company’s financial assets
Marketable securities	$ 300,514,000	$ 0
Assets transferred from level 1 to level 2	0	0
Assets transferred from level 2 to level 1	0	0
Fair Value
Fair value of the Company’s financial assets
Total financial assets	319,855,000
Fair Value \| Quoted Prices in Active Markets (Level 1)
Fair value of the Company’s financial assets
Total financial assets	19,341,000
Fair Value \| Significant Other Observable Inputs (Level 2)
Fair value of the Company’s financial assets
Total financial assets	300,514,000
Money market funds (cash equivalents) \| Fair Value
Fair value of the Company’s financial assets
Money market funds (cash equivalents)	19,341,000	5,000
Money market funds (cash equivalents) \| Fair Value \| Quoted Prices in Active Markets (Level 1)
Fair value of the Company’s financial assets
Money market funds (cash equivalents)	19,341,000	$ 5,000
U.S. treasury securities \| Fair Value
Fair value of the Company’s financial assets
Marketable securities	4,991,000
U.S. treasury securities \| Fair Value \| Significant Other Observable Inputs (Level 2)
Fair value of the Company’s financial assets
Marketable securities	4,991,000
Agency bonds \| Fair Value
Fair value of the Company’s financial assets
Marketable securities	24,451,000
Agency bonds \| Fair Value \| Significant Other Observable Inputs (Level 2)
Fair value of the Company’s financial assets
Marketable securities	24,451,000
Corporate bonds \| Fair Value
Fair value of the Company’s financial assets
Marketable securities	271,072,000
Corporate bonds \| Fair Value \| Significant Other Observable Inputs (Level 2)
Fair value of the Company’s financial assets
Marketable securities	$ 271,072,000

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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).

+ 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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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