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Investments in Marketable Securities - Cost, Gross Unrealized Gains and Losses and Fair Value (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
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Marketable Securities [Abstract] |  |
Cost | $ 128.3 | $ 67.1
Unrealized gains | 15.8 | 13.0
Unrealized losses | 0.0 | (2.3)
Fair value | $ 144.1 | $ 77.8

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- Definition Amount of investment in marketable security. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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