Schedule of Prepaid Expenses, Supplies, Other Assets and Accrued and Other Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Prepaid Expenses, Supplies, and Other Current Assets
Prepaid expenses	$ 8.0	$ 0.7
Supplies	11.6
Restricted cash	3.9
Other	1.2	0.3
Total prepaid expenses, supplies, and other current assets	24.7	1.0
Other Noncurrent Assets
Loans receivable, net	1.0	1.1
Utility and other deposits	6.8
Other	0.4
Total other noncurrent assets	8.2	1.1
Accrued Liabilities
Employee-related costs	13.5	0.8
Accrued interest	14.0
Toll liability	8.2
Income taxes payable	2.0
Environmental liabilities	4.3
Asset retirement obligations	0.9
Derivative liabilities, net	0.7
Other	8.2	6.3
Total accrued liabilities	51.8	7.1
Other Noncurrent Liabilities
Asset retirement obligations	4.1
Other	1.3	0.1
Total other noncurrent liabilities	$ 5.4	$ 0.1