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Marketable Securities and Fair Value Measurements | 6 Months Ended
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Jun. 30, 2021
Marketable Securities and Fair Value Measurements [Abstract] |
Marketable Securities and Fair Value Measurements | 3. Marketable Securities and Fair Value Measurements The following tables present our marketable securities by security type: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | As of June 30, 2021
​ |  | ​ | ​ |  | Gross |  | Gross |  | Estimated
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
(in thousands) | ​ | Cost | ​ | Gains | ​ | Losses | ​ | Value
U.S. treasury securities | ​ | $ | 33,117 | ​ | $ | 4 | ​ | $ | — | ​ | $ | 33,121
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | As of December 31, 2020
​ |  | ​ | ​ |  | Gross |  | Gross |  | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
(in thousands) | ​ | Cost | ​ | Gains | ​ | Losses | ​ | Value
U.S. treasury securities | ​ | $ | 129,446 | ​ | $ | 9 | ​ | $ | (4) | ​ | $ | 129,451
Commercial paper | ​ |  | 52,441 | ​ |  | — | ​ |  | — | ​ |  | 52,441
Corporate debt securities | ​ |  | 17,542 | ​ |  | 4 | ​ |  | — | ​ |  | 17,546
Total | ​ | $ | 199,429 | ​ | $ | 13 | ​ | $ | (4) | ​ | $ | 199,438
​ ​ ​ As of June 30, 2021 and December 31, 2020, marketable securities consisted of investments that mature within one year. The following tables present our assets that are measured at fair value on a recurring basis and indicate the level within the fair value hierarchy of the valuation techniques that we utilized to determine such fair value: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Fair Value Measurements at June 30, 2021 Using:
(in thousands) | ​ | Level 1 | ​ | Level 2 | ​ | Level 3 | ​ | Total
Cash equivalents: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Money market funds | ​ | $ | 226,378 | ​ | $ | — | ​ | $ | — | ​ | $ | 226,378
Marketable securities: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |
U.S. treasury securities | ​ |  | — | ​ |  | 33,121 | ​ |  | — | ​ |  | 33,121
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Fair Value Measurements at December 31, 2020 Using:
(in thousands) |  | Level 1 |  | Level 2 |  | Level 3 |  | Total
Cash equivalents: | ​ | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Money market funds | ​ | $ | 63,827 | ​ | $ | — | ​ | $ | — | ​ | $ | 63,827
Marketable securities: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. treasury securities | ​ | ​ | — | ​ | ​ | 129,451 | ​ | ​ | — | ​ | ​ | 129,451
Commercial paper | ​ | ​ | — | ​ | ​ | 52,441 | ​ | ​ | — | ​ | ​ | 52,441
Corporate debt securities | ​ | ​ | — | ​ | ​ | 17,546 | ​ | ​ | — | ​ | ​ | 17,546
Total | ​ | $ | 63,827 | ​ | $ | 199,438 | ​ | $ | — | ​ | $ | 263,265
​ Money market funds were valued based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy. Our marketable securities, which have consisted of U.S. treasury securities, commercial paper and corporate debt securities were valued using quoted prices in active markets for similar securities, which represent a Level 2 measurement within the fair value hierarchy.

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- Definition The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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