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INVESTMENTS | 12 Months Ended
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Dec. 31, 2022
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, 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 | ​ | $ | —
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | Continuous | ​ | Continuous
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | Unrealized | ​ | Unrealized
​ | ​ | ​ | ​ | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​ | ​ | Loss Position | ​ | Loss Position
​ | ​ | Amortized | ​ | Holding | ​ | Holding | ​ | Fair | ​ | less than 12 | ​ | greater than
December 31, 2021 |  | Cost |  | Gains |  | Losses |  | Value |  | Months |  | 12 Months
Available-for-sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Short-term: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Commercial paper | ​ | $ | 334,077 | ​ | $ | — | ​ | $ | — | ​ | $ | 334,077 | ​ | $ | — | ​ | $ | —
Certificates of deposit | ​ | ​ | 44,502 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 44,502 | ​ | ​ | — | ​ | ​ | —
Municipal securities | ​ |  | 666 | ​ |  | — | ​ |  | — | ​ |  | 666 | ​ |  | — | ​ |  | —
U.S. government agency securities | ​ |  | 62,687 | ​ |  | — | ​ |  | 26 | ​ |  | 62,661 | ​ |  | 26 | ​ |  | —
U.S. treasuries | ​ | ​ | 1,308,536 | ​ | ​ | 2 | ​ | ​ | 717 | ​ | ​ | 1,307,821 | ​ | ​ | 717 | ​ | ​ | —
Long-term: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. government agency securities | ​ | ​ | 12,500 | ​ | ​ | — | ​ | ​ | 24 | ​ | ​ | 12,476 | ​ | ​ | 24 | ​ | ​ | —
U.S. treasuries | ​ | ​ | 87,133 | ​ | ​ | — | ​ | ​ | 190 | ​ | ​ | 86,943 | ​ | ​ | 190 | ​ | ​ | —
Total | ​ | $ | 1,850,101 | ​ | $ | 2 | ​ | $ | 957 | ​ | $ | 1,849,146 | ​ | $ | 957 | ​ | $ | —
​ During the years ended December 31, 2022, 2021 and 2020, realized gains or losses recognized on the sale of investments were not significant. The Company’s investments at December 31, 2022 and 2021 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, 2022 | ​ | December 31, 2021
​ |  | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value
Less than 1 year: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Commercial paper | ​ | $ | 197,712 | ​ | $ | 197,710 |  | $ | 334,077 | ​ | $ | 334,077
Municipal securities | ​ |  | 211,791 | ​ |  | 211,239 |  |  | 666 | ​ |  | 666
U.S. government agency securities | ​ |  | 109,697 | ​ |  | 108,985 |  |  | 62,687 | ​ |  | 62,661
Certificates of deposit | ​ |  | 10,078 | ​ |  | 10,078 |  |  | 44,502 | ​ |  | 44,502
U.S. treasuries | ​ | ​ | 838,825 | ​ | ​ | 834,302 | ​ | ​ | 1,308,536 | ​ | ​ | 1,307,821
Due 1 - 10 years: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. treasuries | ​ | ​ | 53,215 | ​ | ​ | 53,164 | ​ | ​ | 87,133 | ​ | ​ | 86,943
U.S. government agency securities | ​ |  | 2,016 | ​ |  | 2,013 |  |  | 12,500 | ​ |  | 12,476
Variable rate demand notes | ​ | ​ | 4,862 | ​ | ​ | 4,862 | ​ | ​ | — | ​ | ​ | —
Due 11 - 20 years: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Variable rate demand notes | ​ |  | 1,404 | ​ |  | 1,404 |  |  | — | ​ |  | —
Total | ​ | $ | 1,429,600 | ​ | $ | 1,423,757 |  | $ | 1,850,101 | ​ | $ | 1,849,146
​

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- Definition The entire disclosure for investments in certain debt and equity securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -URI https://asc.fasb.org/subtopic&trid=2209399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -URI https://asc.fasb.org/subtopic&trid=2324412 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI https://asc.fasb.org/topic&trid=2196928 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -URI https://asc.fasb.org/subtopic&trid=2176304 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher SEC -Name Regulation S-K (SK) -Number 229 -Section 1403 -Paragraph (b) + Details
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