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Marketable securities (Tables) | 12 Months Ended
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Dec. 31, 2023
Marketable securities |
Schedule of Marketable Securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | Cost or | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | amortized | ​ | Unrealized
​ | ​ | Market value | ​ | cost | ​ | loss, net
​ | ​ | (In millions)
December 31, 2022: |  | ​ |  |  | ​ |  |  | ​ |
Current assets | ​ | $ | 75.1 | ​ | $ | 75.7 | ​ | $ | (.6)
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Noncurrent assets | ​ | $ | 1.2 | ​ | $ | 1.2 | ​ | $ | —
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
December 31, 2023: | ​ |  |  | ​ |  |  | ​ |  |
Current assets | ​ | $ | 56.1 | ​ | $ | 56.1 | ​ | $ | —
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Noncurrent assets | ​ | $ | 4.8 | ​ | $ | 5.0 | ​ | $ | (.2)
Schedule of Marketable Securities and Fair Value Measurements | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Fair Value Measurements
​ | ​ | ​ | ​ | ​ | Quoted | ​ | Significant
​ | ​ | ​ | ​ | ​ | Prices in | ​ | Other
​ | ​ | ​ | ​ | ​ | Active | ​ | Observable
​ | ​ | ​ | ​ | ​ | Markets | ​ | Inputs
​ |  | Total |  | (Level 1) |  | (Level 2)
​ | ​ | (In millions)
December 31, 2022: |  | ​ |  |  | ​ |  |  | ​ |
Current assets: | ​ |  |  | ​ |  |  | ​ |  |
Fixed income securities | ​ | $ | 73.3 | ​ | $ | — | ​ | $ | 73.3
Mutual funds | ​ |  | 1.8 | ​ |  | 1.8 | ​ |  | —
Total | ​ | $ | 75.1 | ​ | $ | 1.8 | ​ | $ | 73.3
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Noncurrent assets - fixed income securities | ​ | $ | 1.2 | ​ | $ | — | ​ | $ | 1.2
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
December 31, 2023: | ​ |  |  | ​ |  |  | ​ |  |
Current assets - fixed income securities | ​ | $ | 56.1 | ​ | $ | — | ​ | $ | 56.1
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Noncurrent assets: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Fixed income securities | ​ | $ | 2.6 | ​ | $ | — | ​ | $ | 2.6
Mutual funds | ​ | ​ | 2.2 | ​ | ​ | 2.2 | ​ |  | —
Total | ​ | $ | 4.8 | ​ | $ | 2.2 | ​ | $ | 2.6
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​

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- Definition Tabular disclosure of assets measured at fair value measured on a recurring or nonrecurring basis. Includes, but is not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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- Definition Tabular disclosure of marketable securities. This may consist of investments in certain debt and equity securities, short-term investments and other assets. + References No definition available. + Details
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