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Marketable Securities - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended	9 Months Ended
	May 30, 2018	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Schedule Of Available For Sale Securities [Line Items]
Securities transaction gain recognized			$ 12.6	$ 0.1
Sale of investment in Amalgamated Sugar Company LLC			250.0
Deemed repayment of snake river sugar company indebtedness			250.0
Snake River
Schedule Of Available For Sale Securities [Line Items]
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