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Fair Value Measurement
6 Months Ended
Jun. 30, 2021
Fair Value Disclosures [Abstract]  
Fair Value Measurement Fair Value Measurement
The following table presents information about our assets and liabilities that are measured at fair value on a recurring basis and indicates the level of the fair value hierarchy and the valuation techniques we utilized to determine such fair value:
As of June 30, 2021
Level 1Level 2Level 3Total
Assets:
Corporate notes and bonds$— $403,551 $— $403,551 
Corporate commercial paper— 487,430 — 487,430 
Obligations of U.S. government agencies— 5,881 — 5,881 
Money market funds138,289 — — 138,289 
Total assets$138,289 $896,862 $— $1,035,151 
Liabilities:
Development derivative liability$— $— $11,607 $11,607 
As of December 31, 2020
Level 1Level 2Level 3Total
Assets:
Corporate notes and bonds$— $687,469 $— $687,469 
Corporate commercial paper— 313,497 — 313,497 
Obligations of U.S. government agencies— 2,382 — 2,382 
Money market funds179,302 — — 179,302 
Total assets179,302 $1,003,348 $— $1,182,650 
Liabilities:
Development derivative liability$— $— $— $— 
Level 1 —    Quoted prices in active markets for identical assets or liabilities.

Level 2 —    Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices for identical or similar assets or liabilities in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.
    
    We use a market approach to value our Level 2 investments. The disclosed fair value related to our investments is based on market prices from a variety of industry standard data providers and generally represents quoted prices for similar assets in active markets or has been derived from observable market data. For the six months ended June 30, 2021, there were no transfers between Level 1 and Level 2 of the fair value hierarchy.
    
Level 3 —    Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. For the reconciliation of the changes in the development derivative liability and the description of the significant inputs used in estimating its fair value, see Note 4.