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Investments in Marketable Securities - Cost, Gross Unrealized Gains and Losses and Fair Value (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Marketable Securities [Abstract] |  |  |  |
Cost |  |  | $ 74.2 | $ 67.1
Unrealized gains | $ 9.0 | $ 13.0 |  |
Unrealized losses | (2.2) | (2.3) |  |
Fair value | $ 81.0 | $ 77.8 | $ 77.8 |

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