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Fair Value Measurements
 3 Months Ended
Mar. 31, 2021
Fair Value Disclosures [Abstract]  
Fair Value Measurements Fair Value Measurements
Fair value is defined as the exchange price that would be received from sale of an asset or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date.
Assets and liabilities measured at fair value are classified into the following categories:
•Level I: Observable inputs are unadjusted quoted prices in active markets for identical assets or liabilities;
•Level II: Observable inputs are quoted prices for similar assets and liabilities in active markets or inputs other than quoted prices that are observable for the assets or liabilities, either directly or indirectly through market corroboration, for substantially the full term of the financial instruments; and
•Level III: Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. These inputs are based on the Company’s own assumptions used to measure assets and liabilities at fair value and require significant management judgment or estimation.
The Company's cash equivalents are comprised of highly liquid money market funds and commercial paper. The Company classifies money market funds within Level I of the fair value hierarchy because they are valued based on quoted market prices in active markets. The Company classifies its investments, which are comprised of U.S. treasury securities, U.S. government agency securities, commercial paper, and corporate bonds, within Level II of the fair value hierarchy because the fair value of these securities is priced by using inputs based on non-binding market consensus prices that are primarily corroborated by observable market data or quoted market prices for similar instruments. The Company recognizes transfers between levels within the fair value hierarchy, if any, at the end of each period. There were no transfers between levels during the periods presented.
The following table summarizes the Company’s cash and available-for-sale securities’ amortized cost, unrealized gains (losses), and fair value by significant investment category reported as cash and cash equivalents, restricted cash, or available-for-sale securities as of March 31, 2021 and December 31, 2020.
(in thousands)    Reported as:
March 31, 2021Amortized
Cost		Unrealized
Gain		Unrealized
(Loss)		Fair ValueCash &
Cash
Equivalents		Available-for-sale securitiesRestricted
Cash
Cash$42,723 $— $— $42,723 $40,064 $— $2,659 
Level I:
Money market funds
154,072 — — 154,072 147,412 — 6,660 
Level II:
Corporate bonds
151,162 17 (75)151,104 — 151,104 — 
U.S. treasury securities
501,018 174 (12)501,180 — 501,180 — 
U.S. government agency securities
15,565 6 — 15,571 — 15,571 — 
Commercial paper
179,893 — — 179,893 — 179,893 — 
Subtotal
847,638 197 (87)847,748 — 847,748 — 
Total assets measured at fair value on a recurring basis
$1,044,433 $197 $(87)$1,044,543 $187,476 $847,748 $9,319 
(in thousands)Reported as:
December 31, 2020Amortized
Cost		Unrealized
Gain		Unrealized
(Loss)		Fair
Value		Cash &
Cash
Equivalents		Available-for-sale securitiesRestricted
Cash
Cash$22,114 $— $— $22,114 $19,523 $— $2,591 
Level I:
Money market funds
71,038 — — 71,038 64,378 — 6,660 
Level II:
Corporate bonds
169,324 43 (26)169,341 — 169,341 — 
U.S. treasury securities
576,652 223 (4)576,871 — 576,871 — 
U.S. government agency securities
15,617 4 (1)15,620 — 15,620 — 
Commercial paper
186,363 — — 186,363 24,994 161,369 — 
Subtotal
947,956 270 (31)948,195 24,994 923,201 — 
Total assets measured at fair value on a recurring basis
$1,041,108 $270 $(31)$1,041,347 $108,895 $923,201 $9,251 
As of March 31, 2021 and December 31, 2020, the Company had $6.7 million in the long-term restricted cash related to irrevocable standby letters of credit established according to the requirements under lease agreements.
The aggregate fair value of the Company’s money market funds approximated amortized cost and, as such, there were no unrealized gains or losses on money market funds as of March 31, 2021 and December 31, 2020. Realized gains and losses, net of tax, were not material for any of the periods presented.
The amortized cost of available-for-sale investments with maturities less than one year was $784.3 million and $866.5 million as of March 31, 2021 and December 31, 2020, respectively. The amortized cost of available-for-sale investments with maturities greater than one year was $63.3 million and $56.5 million as of March 31, 2021 and December 31, 2020, respectively.
As of March 31, 2021, net unrealized gains on investments were not material net of tax and were included in accumulated other comprehensive income on the condensed consolidated balance sheets. As of December 31, 2020, net unrealized gains on investments were $0.2 million net of tax and were included in accumulated other comprehensive income on the condensed consolidated balance sheets. The unrealized gains and losses on available-for-sale investments are related to U.S. treasury securities, U.S. government agency securities, commercial paper, and corporate bonds. The Company determined any unrealized losses to be temporary. Factors considered in determining whether a loss is temporary include the financial condition and near-term prospects of the investee, the extent of the loss related to the credit of the issuer, the expected cash flows from the security, the Company’s intent to sell the security, and whether or not the Company will be required to sell the security before the recovery of its amortized cost. As of March 31, 2021, the Company's investment portfolio consisted of investment grade securities with an average credit rating of AA.
The Company carries the 0.75% Convertible Senior Notes due May 2025 (the Notes) issued in May 2020 at face value less the unamortized discount and issuance costs on its condensed consolidated balance sheets and presents that fair value for disclosure purposes only. As of March 31, 2021, the fair value of the Notes was $1,154.3 million. The fair value of the Notes, which are classified as Level II financial instruments, was determined based on the quoted bid prices of the Notes in an over-the-counter market on the last trading day of the reporting period. For further details on the Notes, refer to Note 7 to these condensed consolidated financial statements.
The Company classifies financial instruments in Level III of the fair value hierarchy when there is reliance on at least one significant unobservable input to the valuation model. In addition to these unobservable inputs, the valuation models for Level III financial instruments typically also rely on a number of inputs that are readily observable, either directly or indirectly. The Company’s assessment of the significance of a particular input to the fair value measurement in its entirety requires management to make judgments and consider factors specific to the asset or liability. There were no financial instruments classified as Level III of the fair value hierarchy as of March 31, 2021 and December 31, 2020.