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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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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.

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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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Fair Value of Financial Instruments - (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair value of the Company’s financial assets
Marketable securities	$ 174,147
Assets transferred from level 1 to level 2	0
Assets transferred from level 2 to level 1	0
Carrying Amount
Fair value of the Company’s financial assets
Total financial assets	179,670
Fair Value
Fair value of the Company’s financial assets
Total financial assets	179,612
Fair Value \| Quoted Prices in Active Markets (Level 1)
Fair value of the Company’s financial assets
Total financial assets	5,465
Fair Value \| Significant Other Observable Inputs (Level 2)
Fair value of the Company’s financial assets
Total financial assets	174,147
Money market funds (cash equivalents) \| Carrying Amount
Fair value of the Company’s financial assets
Money market funds (cash equivalents)	5,465	$ 5,000
Money market funds (cash equivalents) \| Fair Value
Fair value of the Company’s financial assets
Money market funds (cash equivalents)	5,465	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)	5,465	$ 5,000
U.S. treasury securities \| Carrying Amount
Fair value of the Company’s financial assets
Marketable securities	19,982
U.S. treasury securities \| Fair Value
Fair value of the Company’s financial assets
Marketable securities	19,983
U.S. treasury securities \| Fair Value \| Significant Other Observable Inputs (Level 2)
Fair value of the Company’s financial assets
Marketable securities	19,983
Agency bonds \| Carrying Amount
Fair value of the Company’s financial assets
Marketable securities	47,178
Agency bonds \| Fair Value
Fair value of the Company’s financial assets
Marketable securities	47,161
Agency bonds \| Fair Value \| Significant Other Observable Inputs (Level 2)
Fair value of the Company’s financial assets
Marketable securities	47,161
Corporate bonds \| Carrying Amount
Fair value of the Company’s financial assets
Marketable securities	107,045
Corporate bonds \| Fair Value
Fair value of the Company’s financial assets
Marketable securities	107,003
Corporate bonds \| Fair Value \| Significant Other Observable Inputs (Level 2)
Fair value of the Company’s financial assets
Marketable securities	$ 107,003