Marketable Securities - Schedule of Marketable Securities and Fair Value Measurements (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, current	$ 3.0	$ 4.4
Marketable securities, noncurrent	255.7	253.5
Fixed Income Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, noncurrent	4.4	2.9
Mutual Funds and Common Stocks
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, noncurrent		0.6
Common Stocks and Exchange Traded Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, noncurrent	1.3
Amalgamated Sugar Company LLC
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, noncurrent	250.0	250.0
Quoted Prices in Active Markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, noncurrent	1.3	0.6
Quoted Prices in Active Markets (Level 1) | Mutual Funds and Common Stocks
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, noncurrent		0.6
Quoted Prices in Active Markets (Level 1) | Common Stocks and Exchange Traded Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, noncurrent	1.3
Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, current	3.0	4.4
Marketable securities, noncurrent	4.4	2.9
Significant Other Observable Inputs (Level 2) | Fixed Income Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, noncurrent	4.4	2.9
Significant Unobservable Inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, noncurrent	250.0	250.0
Significant Unobservable Inputs (Level 3) | Amalgamated Sugar Company LLC
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, noncurrent	$ 250.0	$ 250.0