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Marketable Securities - Schedule of Marketable Securities (Detail) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Market value, available for sale securities, current $ 2.2 $ 3.0
Market value, available for sale securities, noncurrent 5.1 255.7
Non Current Assets    
Schedule Of Available For Sale Securities [Line Items]    
Cost basis 5.4 255.9
Unrealized losses, net (0.3) (0.2)
Current Assets    
Schedule Of Available For Sale Securities [Line Items]    
Cost basis 2.2 3.0
Unrealized losses, net 0.0 0.0
Other | Non Current Assets    
Schedule Of Available For Sale Securities [Line Items]    
Market value, available for sale securities, noncurrent   5.7
Cost basis   5.9
Unrealized losses, net   (0.2)
Amalgamated Sugar Company LLC    
Schedule Of Available For Sale Securities [Line Items]    
Market value, available for sale securities, noncurrent $ 250.0  
Amalgamated Sugar Company LLC | Non Current Assets    
Schedule Of Available For Sale Securities [Line Items]    
Market value, available for sale securities, noncurrent   250.0
Cost basis   250.0
Unrealized losses, net   $ 0.0