Accrued Liabilities - Components of Accrued Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Compensation and benefits	$ 13.7	$ 25.1
Deferred sales revenue - toll customers	8.3	4.0
Workers’ compensation	3.0	3.0
Utilities	2.5	2.0
Professional fees	2.3	1.8
Insurance	2.0	3.1
Taxes	1.1	1.3
Other	8.9	4.7
Total accrued liabilities	$ 41.8	$ 45.0