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Investments in Marketable Securities (Tables) | 12 Months Ended
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Dec. 31, 2020
Marketable Securities [Abstract] |
Schedule of Cost, Gross Unrealized Gains and Losses, and Fair Value of Investments | The following is a summary of the cost, gross unrealized gains, unrealized losses, and fair value of Investments in marketable securities:
|  | December 31,
|  | 2019 |  | 2020
Cost |  | $ | 57.9 |  |  | $ | 69.4 |
Unrealized gains |  | 2.1 |  |  | 5.5 |
Unrealized losses |  | (0.6) |  |  | (0.0) |
Fair value |  | $ | 59.4 |  |  | $ | 74.9 |

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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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