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Fair Value Measurements
 3 Months Ended
Mar. 31, 2024
Fair Value Disclosures [Abstract]  
Fair Value Measurements Fair Value Measurements
The Company measures its financial instruments at fair value each reporting period using a fair value hierarchy that prioritizes the use of observable inputs and minimizes the use of unobservable inputs when measuring fair value. A financial instrument’s classification within the fair value hierarchy is based upon the lowest level of input that is significant to the fair value measurement.

The following table presents information about the Company’s financial instruments that are measured at fair value on a recurring basis using the input categories discussed in Note 1:   
As of March 31, 2024
Level 1Level 2Level 3Total
Cash equivalents
Money market funds$439.2 $— $— $439.2 
Total cash equivalents$439.2 $— $— $439.2 
Short-term investments
Corporate notes and obligations— 328.1 — 328.1 
U.S. Treasury securities— 157.8 — 157.8 
Asset backed securities— 59.2 — 59.2 
Municipal securities— 41.1 — 41.1 
Commercial paper— 27.0 — 27.0 
Certificates of deposit— 6.7 — 6.7 
U.S. agency obligations— 3.5 — 3.5 
Foreign government obligations— 1.9 — 1.9 
Supranational securities— 1.7 — 1.7 
Total short-term investments— 627.0 — 627.0 
Total $439.2 $627.0 $— $1,066.2 
As of December 31, 2023
Level 1Level 2Level 3Total
Cash equivalents
Money market funds$514.8 $— $— $514.8 
U.S. Treasury securities— 10.0 — 10.0 
Commercial paper— 4.4 — 4.4 
Corporate notes and obligations— 2.9 — 2.9 
Certificates of deposit— 1.3 — 1.3 
Municipal securities— 0.2 — 0.2 
Total cash equivalents$514.8 $18.8 $— $533.6 
Short-term investments
Corporate notes and obligations— 349.7 — 349.7 
U.S. Treasury securities— 225.3 — 225.3 
Asset backed securities— 70.0 — 70.0 
Municipal securities— 46.3 — 46.3 
Commercial paper— 30.7 — 30.7 
Certificates of deposit— 8.4 — 8.4 
U.S. agency obligations— 5.7 — 5.7 
Foreign government obligations— 3.3 — 3.3 
Supranational securities— 1.7 — 1.7 
Total short-term investments— 741.1 — 741.1 
Total$514.8 $759.9 $— $1,274.7 

The Company had no transfers between levels of the fair value hierarchy during the periods presented.

The carrying amounts of certain financial instruments, including cash held in banks, accounts receivable and accounts payable approximate fair value due to their short-term maturities and are excluded from the fair value table above.

The Company had $695.8 million in aggregate principal amount of 0% convertible senior notes due in 2026 (the "2026 Notes"), and $693.3 million in aggregate principal amount of 0% convertible senior notes due in 2028 (the "2028 Notes" and together with the 2026 Notes, the "Notes"), outstanding as of March 31, 2024. Refer to Note 8 "Debt" for further details on the 2026 Notes and 2028 Notes.

The estimated fair value of the 2026 Notes and the 2028 Notes, based on a market approach as of March 31, 2024 was approximately $674.9 million and $649.3 million, respectively. The Notes were categorized as Level 2 instruments as the estimated fair value was determined based on the estimated or actual bids and offers of the Notes in an over-the-counter market on the last business day of the period.