Prepaid Expenses and Other Current Assets - Components of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Deferred cost of sales - toll customers	$ 8.3	$ 4.0
Workers’ compensation plan deposits	4.1	6.3
Derivative contract assets	3.4	2.8
Prepaid insurance	1.9	1.7
Other	3.7	3.5
Total prepaid expenses and other current assets	$ 21.4	$ 18.3