Marketable Securities - Schedule of Marketable Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Market value, available for sale securities, current	$ 1.6	$ 3.0
Market value, available for sale securities, noncurrent	256.8	255.7
Non Current Assets
Schedule Of Available For Sale Securities [Line Items]
Cost basis	257.1	255.9
Unrealized losses, net	(0.3)	(0.2)
Current Assets
Schedule Of Available For Sale Securities [Line Items]
Cost basis	1.6	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	6.8	5.7
Cost basis	7.1	5.9
Unrealized losses, net	(0.3)	(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	250.0
Cost basis	250.0	250.0
Unrealized losses, net	$ 0.0	$ 0.0