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Investments in Marketable Securities - Summary of the Cost, Gross Unrealized Gains and Losses, and Fair Value of Investments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Marketable Securities [Abstract]
Cost	$ 126.8	$ 67.1
Unrealized gains	1.1	13.0
Unrealized losses	(8.6)	(2.3)
Fair value	$ 119.3	$ 77.8

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- Definition

Marketable Securities, Cost

+ References

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- Definition

Marketable Securities, Unrealized Gains

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- Definition

Marketable Securities, Unrealized Losses

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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

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