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Marketable Securities - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	May 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Marketable securities			$ 4.8	$ 255.7
Securities transaction gain recognized			12.4	0.1	$ 0.5
Sale of investment in Amalgamated Sugar Company LLC			250.0
Deemed repayment of snake river sugar company indebtedness			250.0
Amalgamated Sugar Company LLC
Schedule Of Available For Sale Securities [Line Items]
Net of assets transferred to the LLC				34.0
Marketable securities				250.0
Cost of the investment in LLC				$ 250.0
Percentage of share from profit of LLC				95.00%
Amalgamated Sugar Company LLC \| Base Level
Schedule Of Available For Sale Securities [Line Items]
Distributions from LLC - aggregate annual base level				$ 26.7
Snake River
Schedule Of Available For Sale Securities [Line Items]
Loan issued by Snake River				$ 250.0
Securities transaction gain recognized	$ 12.5	$ 12.5	12.5
Sale of investment in Amalgamated Sugar Company LLC	$ 250.0
Deemed repayment of snake river sugar company indebtedness		$ 250.0
Snake River \| Amalgamated Sugar Company LLC
Schedule Of Available For Sale Securities [Line Items]
Minimum amount annual distributions received from Amalgamated Sugar LLC will exceed Snake River loan interest payments			$ 1.8