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Cash and Cash Equivalents, Short-term Investments and Restricted Cash | 6 Months Ended
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Jun. 30, 2021
Cash and Cash Equivalents [Abstract] |
Cash and Cash Equivalents, Short-term Investments and Restricted Cash | Cash and Cash Equivalents, Short-term Investments and Restricted CashThe following tables present the amortized cost, gross unrealized gains and losses and estimated fair market value of our cash and cash equivalents, short-term investments and restricted cash as of the dates presented (in thousands):
| June 30, 2021
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Market Value
Cash | $ | 218,514 |  |  | $ | — |  |  | $ | — |  |  | $ | 218,514 |
Cash equivalents: |  |  |  |  |  |  |
Money market funds | 3,494,642 |  |  | — |  |  | — |  |  | 3,494,642 |
Short-term investments: |  |  |  |  |  |  |
U. S. government agency securities | 829,791 |  |  | 134 |  |  | — |  |  | 829,925 |
Treasury bills | 44,975 |  |  | 13 |  |  | — |  |  | 44,988 |
Municipal securities | 5,962 |  |  | — |  |  | — |  |  | 5,962 |
Restricted cash | 141,024 |  |  | — |  |  | — |  |  | 141,024 |
Total | $ | 4,734,908 |  |  | $ | 147 |  |  | $ | — |  |  | $ | 4,735,055 |
| December 31, 2020
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Market Value
Cash | $ | 213,518 |  |  | $ | — |  |  | $ | — |  |  | $ | 213,518 |
Cash equivalents: |  |  |  |  |  |  |
Money market funds | 1,486,384 |  |  | — |  |  | — |  |  | 1,486,384 |
Municipal securities | 3,229 |  |  | — |  |  | (1) |  |  | 3,228 |
Short-term investments: |  |  |  |  |  |  |
Treasury bills | 1,163,748 |  |  | 65 |  |  | — |  |  | 1,163,813 |
U.S. government agency securities | 1,037,572 |  |  | 57 |  |  | (52) |  |  | 1,037,577 |
Municipal securities | 16,226 |  |  | — |  |  | (6) |  |  | 16,220 |
Certificates of deposit | 498 |  |  | — |  |  | — |  |  | 498 |
Restricted cash | 75,805 |  |  | — |  |  | — |  |  | 75,805 |
Total | $ | 3,996,980 |  |  | $ | 122 |  |  | $ | (59) |  |  | $ | 3,997,043 |
All short-term investments as of June 30, 2021 have a contractual maturity date of one year or less.

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- Definition The entire disclosure of the components of cash, cash equivalents, and short-term investments. Short-term investments may include current marketable securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 + Details
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