Prepaid Expenses and Other Current Assets - Components of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Workers’ compensation plan deposits	$ 7.2	$ 6.2	$ 6.2
Deferred cost of sales - toll customers	6.2	12.7	8.6
Prepaid insurance	3.8	2.8	5.2
Derivative contract assets	1.7	1.3	1.5
Prepaid tooling	0.7	1.2	1.1
Other	2.7	2.0	3.1
Total prepaid expenses and other current assets	$ 22.3	$ 26.2	$ 25.7