Schedule of Prepaid Expenses, Supplies, Other Assets and Accrued and Other Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Prepaid Expenses, Supplies and Other Current Assets
Prepaid expenses	$ 8.6	$ 8.0
Inventory supplies	11.8	11.6
Income taxes receivable	2.4	0.9
Restricted cash and restricted cash equivalents	1.0	3.9
Derivative assets	0.6
Other	0.2	0.3
Total prepaid expenses, supplies and other current assets	24.6	24.7
Other Noncurrent Assets
Loans receivable, net	0.8	1.0
Utility and other deposits	4.4	3.2
Restricted cash and restricted cash equivalents	4.5	3.6
Other	0.1	0.4
Total other noncurrent assets	9.8	8.2
Accrued Liabilities
Employee-related costs	11.2	13.5
Accrued interest	14.1	14.0
Toll liability	6.7	8.2
Income taxes payable	5.3	2.0
Environmental liabilities	4.0	4.3
Asset Retirement Obligation, current	0.9	0.9
Derivative liabilities	0.4	0.7
Other	3.8	8.2
Total accrued liabilities	46.4	51.8
Other Noncurrent Liabilities
Asset Retirement Obligation, non-current	4.4	4.1
Other	2.5	1.3
Total other noncurrent liabilities	$ 6.9	$ 5.4