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Cash and Investments in Marketable Securities
12 Months Ended
Dec. 31, 2020
Cash and Cash Equivalents [Abstract]  
Cash and Investments in Marketable Securities Cash and Investments in Marketable Securities
    Cash and investments in marketable securities, including cash equivalents, are as follows (in thousands):
Estimated Fair Value at
December 31,
2020
December 31,
2019
Cash and cash equivalents$198,955 $96,363 
Short-term investments862,941 1,228,499 
Long-term investments136,662 279,119 
Total cash and investments in marketable securities$1,198,558 $1,603,981 
    We invest in liquid, high quality debt securities. Our investments in debt securities are subject to interest rate risk. To minimize the exposure due to an adverse shift in interest rates, we invest in securities with maturities of two years or less and maintain a weighted average maturity of one year or less. All of our long-term investments as of December 31, 2020 and 2019 had maturities between one and two years.
    During the year ended December 31, 2020 and 2018, we sold available-for-sale securities totaling $41.7 million and $12.0 million, respectively. Gross realized gains and losses on those sales were not significant. During the year ended December 31, 2019, we did not sell any of our available-for-sale securities. The cost of securities sold is based on the specific identification method.
We report our accrued interest receivable, which totaled $5.1 million and $6.5 million at December 31, 2020 and December 31, 2019, respectively, in other current assets on our Consolidated Balance Sheets.
    Our portfolio of cash and investments in marketable securities includes (in thousands):
Fair Value
Hierarchy
Level
December 31, 2020December 31, 2019
Amortized CostGross Unrealized GainsGross Unrealized LossesFair ValueFair Value
Corporate notes and bonds2$686,543 $1,035 $(109)$687,469 $1,132,182 
Corporate commercial paper2313,520 44 (67)313,497 375,473 
Obligations of U.S. government agencies22,380 — 2,382 — 
Available-for-sale investments1,002,443 1,081 (176)1,003,348 1,507,655 
Money market funds1179,302 83,546 
Certificate of depositN/A9,623 6,951 
CashN/A6,285 5,829 
Total cash and investments in marketable securities$1,198,558 $1,603,981 
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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 years ended December 31, 2020 and 2019, 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.
    At December 31, 2019, our gross unrealized gains and losses totaled $2.1 million and $0.3 million, respectively.
At December 31, 2020 and 2019, we had letter of credit arrangements in favor of our landlords and certain vendors totaling $8.1 million and $6.8 million, respectively. These letters of credit are secured by investments of similar amounts.