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Investments in Marketable Securities - Cost, Gross Unrealized Gains and Losses and Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Marketable Securities [Abstract]
Cost	$ 54.0	$ 69.4
Unrealized gains	9.3	5.5
Unrealized losses	(0.5)	(0.0)
Fair value	$ 62.8	$ 74.9

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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=120398452&loc=d3e534808-122878

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