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Investments in Marketable Securities (Tables)
 12 Months Ended
Dec. 31, 2018
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 and losses and fair value of Investments in marketable securities:
 
 
December 31,
 
 
2017
 
2018
Cost
 
$
67.1

 
$
126.8

Unrealized gains
 
13.0

 
1.1

Unrealized losses
 
(2.3
)
 
(8.6
)
Fair value
 
$
77.8

 
$
119.3