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INVESTMENTS | 12 Months Ended
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Dec. 31, 2023
INVESTMENTS |
INVESTMENTS | 5. 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
December 31, 2023 |  | Cost |  | Gains |  | Losses |  | Value |  | Months |  | 12 Months
Available-for-sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Short-term: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Commercial paper | ​ | $ | 163,775 | ​ | $ | — | ​ | $ | 1 | ​ | $ | 163,774 | ​ | $ | 1 | ​ | $ | —
Certificates of deposit | ​ | ​ | 15,590 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 15,590 | ​ | ​ | — | ​ | ​ | —
Municipal securities | ​ |  | 361 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 361 | ​ | ​ | — | ​ | ​ | —
U.S. government agency securities | ​ |  | 116,524 | ​ |  | 90 | ​ |  | 66 | ​ |  | 116,548 | ​ |  | 66 | ​ |  | —
U.S. treasuries | ​ | ​ | 412,936 | ​ |  | 205 | ​ |  | 1,084 | ​ |  | 412,057 | ​ |  | 1,084 | ​ |  | —
Corporate bonds | ​ | ​ | 247,340 | ​ | ​ | 89 | ​ | ​ | 154 | ​ | ​ | 247,275 | ​ | ​ | 154 | ​ | ​ | —
Long-term: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. government agency securities | ​ | ​ | 23,485 | ​ | ​ | 51 | ​ | ​ | 5 | ​ | ​ | 23,531 | ​ | ​ | 5 | ​ | ​ | —
U.S. treasuries | ​ | ​ | 35,896 | ​ | ​ | 79 | ​ | ​ | 8 | ​ | ​ | 35,967 | ​ | ​ | 8 | ​ | ​ | —
Corporate bonds | ​ | ​ | 16,903 | ​ | ​ | 32 | ​ | ​ | 2 | ​ | ​ | 16,933 | ​ | ​ | 2 | ​ | ​ | —
Total | ​ | $ | 1,032,810 | ​ | $ | 546 | ​ | $ | 1,320 | ​ | $ | 1,032,036 | ​ | $ | 1,320 | ​ | $ | —
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | Continuous | ​ | Continuous
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | Unrealized | ​ | Unrealized
​ | ​ | ​ | ​ | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​ | ​ | Loss Position | ​ | Loss Position
​ | ​ | Amortized | ​ | Holding | ​ | Holding | ​ | Fair | ​ | less than 12 | ​ | greater than
December 31, 2022 |  | Cost |  | Gains |  | Losses |  | Value |  | Months |  | 12 Months
Available-for-sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Short-term: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Commercial paper | ​ | $ | 197,712 | ​ | $ | 1 | ​ | $ | 4 | ​ | $ | 197,709 | ​ | $ | 4 | ​ | $ | —
Certificates of deposit | ​ | ​ | 10,078 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 10,078 | ​ | ​ | — | ​ | ​ | —
Municipal securities | ​ |  | 211,791 | ​ |  | 60 | ​ |  | 612 | ​ |  | 211,239 | ​ |  | 612 | ​ |  | —
U.S. government agency securities | ​ |  | 109,697 | ​ |  | 3 | ​ |  | 715 | ​ |  | 108,985 | ​ |  | 715 | ​ |  | —
U.S. treasuries | ​ | ​ | 838,825 | ​ | ​ | 17 | ​ | ​ | 4,539 | ​ | ​ | 834,303 | ​ | ​ | 4,539 | ​ | ​ | —
Long-term: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. government agency securities | ​ | ​ | 2,016 | ​ | ​ | — | ​ | ​ | 3 | ​ | ​ | 2,013 | ​ | ​ | 3 | ​ | ​ | —
U.S. treasuries | ​ | ​ | 53,215 | ​ | ​ | 20 | ​ | ​ | 71 | ​ | ​ | 53,164 | ​ | ​ | 71 | ​ | ​ | —
Variable rate demand notes | ​ | ​ | 6,266 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 6,266 | ​ | ​ | — | ​ | ​ | —
Total | ​ | $ | 1,429,600 | ​ | $ | 101 | ​ | $ | 5,944 | ​ | $ | 1,423,757 | ​ | $ | 5,944 | ​ | $ | —
​ During the years ended December 31, 2023, 2022 and 2021, realized gains or losses recognized on the sale of investments were not significant. The Company’s investments at December 31, 2023 and 2022 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, 2023 | ​ | December 31, 2022
​ |  | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value
Less than 1 year: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Commercial paper | ​ | $ | 163,775 | ​ | $ | 163,774 |  | $ | 197,712 | ​ | $ | 197,710
Municipal securities | ​ |  | 361 | ​ |  | 361 |  |  | 211,791 | ​ |  | 211,239
U.S. government agency securities | ​ |  | 116,524 | ​ |  | 116,548 |  |  | 109,697 | ​ |  | 108,985
Certificates of deposit | ​ |  | 15,590 | ​ |  | 15,590 |  |  | 10,078 | ​ |  | 10,078
U.S. treasuries | ​ | ​ | 412,936 | ​ | ​ | 412,057 | ​ | ​ | 838,825 | ​ | ​ | 834,302
Corporate bonds | ​ | ​ | 247,340 | ​ | ​ | 247,275 | ​ | ​ | — | ​ | ​ | —
Due 1 - 10 years: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. treasuries | ​ | ​ | 35,896 | ​ | ​ | 35,967 | ​ | ​ | 53,215 | ​ | ​ | 53,164
U.S. government agency securities | ​ |  | 23,485 | ​ |  | 23,531 |  |  | 2,016 | ​ |  | 2,013
Variable rate demand notes | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 4,862 | ​ | ​ | 4,862
Corporate bonds | ​ | ​ | 16,903 | ​ | ​ | 16,933 | ​ | ​ | — | ​ | ​ | —
Due 11 - 20 years: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Variable rate demand notes | ​ |  | — | ​ |  | — |  |  | 1,404 | ​ |  | 1,404
Total | ​ | $ | 1,032,810 | ​ | $ | 1,032,036 |  | $ | 1,429,600 | ​ | $ | 1,423,757
​

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