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INVESTMENTS | 12 Months Ended
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Dec. 31, 2020
INVESTMENTS |
INVESTMENTS | 4. INVESTMENTS The following table summarizes the Company’s investments at: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | Continuous | ​ | Continuous
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | Unrealized | ​ | Unrealized
​ | ​ | ​ | ​ | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​ | ​ | Loss Position | ​ | Loss Position
​ | ​ | Amortized | ​ | Holding | ​ | Holding | ​ | Fair | ​ | less than 12 | ​ | greater than 12
December 31, 2020 |  | Cost |  | Gains |  | Losses |  | Value |  | Months |  | Months
Available-for-sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Short-term: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Commercial paper | ​ | $ | 119,886 | ​ | $ | — | ​ | $ | — | ​ | $ | 119,886 | ​ | $ | — | ​ | $ | —
Certificates of deposit | ​ | ​ | 20,387 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 20,387 | ​ | ​ | — | ​ | ​ | —
Municipal securities | ​ |  | 9,083 | ​ |  | — | ​ |  | — | ​ |  | 9,083 | ​ |  | — | ​ |  | —
U.S. government agency securities | ​ |  | 81,521 | ​ |  | 13 | ​ |  | 3 | ​ |  | 81,531 | ​ |  | 3 | ​ |  | —
U.S. treasuries | ​ | ​ | 650,386 | ​ | ​ | 150 | ​ | ​ | 69 | ​ | ​ | 650,467 | ​ | ​ | 69 | ​ | ​ | —
Long-term: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. government agency securities | ​ | ​ | 10,350 | ​ | ​ | 1 | ​ | ​ | — | ​ | ​ | 10,351 | ​ | ​ | — | ​ | ​ | —
U.S. treasuries | ​ | ​ | 33,946 | ​ | ​ | 1 | ​ | ​ | 7 | ​ | ​ | 33,940 | ​ | ​ | 7 | ​ | ​ | —
Total | ​ | $ | 925,559 | ​ | $ | 165 | ​ | $ | 79 | ​ | $ | 925,645 | ​ | $ | 79 | ​ | $ | —
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | Continuous | ​ | Continuous
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | Unrealized | ​ | Unrealized
​ | ​ | ​ | ​ | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​ | ​ | Loss Position | ​ | Loss Position
​ | ​ | Amortized | ​ | Holding | ​ | Holding | ​ | Fair | ​ | less than 12 | ​ | greater than 12
December 31, 2019 |  | Cost |  | Gains |  | Losses |  | Value |  | Months |  | Months
Available-for-sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Short-term: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Commercial paper | ​ | $ | 83,478 | ​ | $ | — | ​ | $ | — | ​ | $ | 83,478 | ​ | $ | — | ​ | $ | —
Certificates of deposit | ​ | ​ | 28,049 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 28,049 | ​ | ​ | — | ​ | ​ | —
Municipal securities | ​ |  | 147,983 | ​ |  | 145 | ​ |  | 20 | ​ |  | 148,108 | ​ |  | 20 | ​ |  | —
U.S. government agency securities | ​ |  | 40,620 | ​ |  | 5 | ​ |  | 35 | ​ |  | 40,590 | ​ |  | 35 | ​ |  | —
U.S. treasuries | ​ | ​ | 211,055 | ​ | ​ | 134 | ​ | ​ | 31 | ​ | ​ | 211,158 | ​ | ​ | 31 | ​ | ​ | —
Variable rate demand notes | ​ |  | 21,680 | ​ |  | — | ​ |  | — | ​ |  | 21,680 | ​ |  | — | ​ |  | —
Long-term: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Municipal securities | ​ | ​ | 1,562 | ​ | ​ | — | ​ | ​ | 1 | ​ | ​ | 1,561 | ​ | ​ | 1 | ​ | ​ | —
U.S. government agency securities | ​ | ​ | 5,267 | ​ | ​ | — | ​ | ​ | 1 | ​ | ​ | 5,266 | ​ | ​ | 1 | ​ | ​ | —
U.S. treasuries | ​ | ​ | 6,077 | ​ | ​ | 1 | ​ | ​ | — | ​ | ​ | 6,078 | ​ | ​ | — | ​ | ​ | —
Total | ​ | $ | 545,771 | ​ | $ | 285 | ​ | $ | 88 | ​ | $ | 545,968 | ​ | $ | 88 | ​ | $ | —
​ During the years ended December 31, 2020, 2019 and 2018, realized gains or losses recognized on the sale of investments were not significant. The Company’s investments at December 31, 2020 and 2019 carried investment grade credit ratings. Variable rate demand notes (“VRDNs”) are floating rate municipal bonds with embedded put options that allow the bondholder to sell the security at par plus accrued interest. All of the put options are secured by a pledged liquidity source. While they are classified as marketable investment securities, the put option allows the VRDNs to be liquidated at par on a same day, or more generally, on a seven-day settlement basis. The following table summarizes the underlying contractual maturities of the Company’s investments at: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31, 2020 | ​ | December 31, 2019
​ |  | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value
Less than 1 year: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Commercial paper | ​ | $ | 119,886 | ​ | $ | 119,886 |  | $ | 83,478 | ​ | $ | 83,478
Municipal securities | ​ |  | 9,083 | ​ |  | 9,083 |  |  | 147,983 | ​ |  | 148,108
U.S. government agency securities | ​ |  | 81,521 | ​ |  | 81,531 |  |  | 40,620 | ​ |  | 40,590
Certificates of deposit | ​ |  | 20,387 | ​ |  | 20,387 |  |  | 28,049 | ​ |  | 28,049
U.S. treasuries | ​ | ​ | 650,386 | ​ | ​ | 650,467 | ​ | ​ | 211,055 | ​ | ​ | 211,158
Due 1 -10 years: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Municipal securities | ​ |  | — | ​ |  | — |  |  | 1,562 | ​ |  | 1,561
U.S. treasuries | ​ | ​ | 33,946 | ​ | ​ | 33,940 | ​ | ​ | 6,077 | ​ | ​ | 6,078
U.S. government agency securities | ​ |  | 10,350 | ​ |  | 10,351 |  |  | 5,267 | ​ |  | 5,266
Variable rate demand notes | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 3,905 | ​ | ​ | 3,905
Due 11 - 20 years: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Variable rate demand notes | ​ |  | — | ​ |  | — |  |  | 8,886 | ​ |  | 8,886
Due 21 - 30 years: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Variable rate demand notes | ​ |  | — | ​ |  | — |  |  | 6,885 | ​ |  | 6,885
Due 31 - 40 years: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Variable rate demand notes | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 2,004 | ​ | ​ | 2,004
Total | ​ | $ | 925,559 | ​ | $ | 925,645 |  | $ | 545,771 | ​ | $ | 545,968
​

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