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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]          
Marketable securities     $ 6.2 $ 4.8  
Securities transaction gain recognized     0.3 12.4 $ 0.1
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