Prepaid Expenses, Supplies, Other Assets and Accrued and Other Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Prepaid Expenses, Supplies and Other Current Assets
Prepaid expenses	$ 17.4	$ 8.6
Inventory supplies	12.6	11.8
Income taxes receivable	1.9	2.4
Restricted cash and restricted cash equivalents	0.7	1.0
Derivative assets		0.6
Other	1.3	0.2
Total prepaid expenses, supplies and other current assets	33.9	24.6
Other Noncurrent Assets
Loans receivable, net	0.7	0.8
Utility and other deposits	4.9	4.4
Restricted cash and restricted cash equivalents	7.2	4.5
Other		0.1
Other Assets, Noncurrent, Total	12.8	9.8
Accrued Liabilities
Employee-related costs	8.8	11.2
Accrued interest	0.8	14.1
Toll liability	9.8	6.7
Income taxes payable	2.7	5.3
Environmental liabilities	3.3	4.0	$ 4.3
Asset Retirement Obligation, current	0.2	0.9
Derivative liabilities		0.4
Other	2.5	3.8
Accrued Liabilities, Current, Total	28.1	46.4
Other Noncurrent Liabilities
Asset Retirement Obligation, non-current	5.6	4.4
Other	1.8	2.5
Other Liabilities, Noncurrent, Total	$ 7.4	$ 6.9