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Fair Value Measurement
 6 Months Ended
Jun. 30, 2020
Fair Value Disclosures [Abstract]  
Fair Value Measurement Fair Value Measurement
We have established a fair value hierarchy used to determine the fair value of our financial instruments as follows:

Level 1—Inputs are unadjusted quoted prices in active markets for identical assets or liabilities.

Level 2—Inputs are quoted prices for similar assets and liabilities in active markets or inputs that are observable for the assets or liabilities, either directly or indirectly through market corroboration, for substantially the full term of the financial instruments.

Level 3—Inputs are unobservable inputs based on our own assumptions used to measure assets and liabilities at fair value; the inputs require significant management judgment or estimation.

A financial instrument’s classification within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.
Financial instruments measured and recorded at fair value on a recurring basis as of June 30, 2020 and December 31, 2019 are classified based on the valuation technique level in the tables below (in thousands):
 June 30, 2020
 TotalLevel 1Level 2
Assets:   
Cash equivalents:   
U.S. treasury securities$117,402  $117,402  $—  
Money market funds150,002  150,002  —  
Short-term investments: 
Corporate securities402,434  —  402,434  
U.S. treasury securities15,096  15,096  —  
Long-term investments:
Corporate securities219,736  —  219,736  
Agency bonds60,756  —  60,756  
Total assets measured and recorded at fair value$965,426  $282,500  $682,926  

 December 31, 2019
 TotalLevel 1Level 2
Assets:   
Cash equivalents:   
Money market funds$146,165  $146,165  $—  
Short-term investments:
Commercial paper7,489  —  7,489  
Corporate securities319,293  —  319,293  
U.S. treasury securities44,286  44,286  —  
Agency bonds10,006  —  10,006  
Long-term investments:
Corporate securities295,478  —  295,478  
Agency bonds15,005  —  15,005  
Total assets measured and recorded at fair value$837,722  $190,451  $647,271  
 
We value our investments based on quoted prices in active markets for identical assets (Level 1 inputs) or inputs other than quoted prices that are observable either directly or indirectly (Level 2 inputs) in determining fair value. Other than our money market funds and U.S. treasury securities, we classify our fixed income available-for-sale investments as having Level 2 inputs. The valuation techniques used to measure the fair value of our financial instruments having Level 2 inputs were derived from non-binding market consensus prices that are corroborated by observable market data, quoted market prices for similar instruments, or pricing models such as discounted cash flow techniques. We do not hold any investments valued with a Level 3 input.

The methods described above may produce a fair value calculation that may not be indicative of net realizable value or reflective of future fair values. Furthermore, while we believe our valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date.
 
Financial Instruments Not Recorded at Fair Value on a Recurring Basis

We report our financial instruments at fair value with the exception of the notes. The estimated fair value of the notes was determined based on the trading price of the notes as of the last day of trading for the period. We consider the fair value of the notes to be a Level 2 measurement due to the limited trading activity. For further information on the notes see Note 8, “Convertible Senior Notes.”
The carrying amounts and estimated fair values of the notes as of June 30, 2020 and December 31, 2019 are as follows (in thousands):
June 30, 2020December 31, 2019
 Carrying AmountEstimated Fair ValueCarrying AmountEstimated Fair Value
2025 notes$621,508  $1,135,320  $602,611  $831,000  
2023 notes304,685  862,840  297,692  523,538  
Convertible senior notes, net$926,193  $1,998,160  $900,303  $1,354,538  
The carrying amount of the 2025 notes and 2023 notes as of June 30, 2020 was net of unamortized debt discount of $167.0 million and $36.0 million, respectively, and unamortized issuance costs of $11.5 million and $4.3 million, respectively. The carrying amount of the 2025 notes and 2023 notes as of December 31, 2019 was net of unamortized debt discount of $184.7 million and $42.3 million, respectively, and unamortized issuance costs of $12.7 million and $5.0 million, respectively.