| In millions, except per share items | Sep-30-18 | Jun-30-18 | Mar-31-18 | Dec-31-17 | Sep-30-17 | Jun-30-17 | Mar-31-17 | Dec-31-16 |
| | 10-Q | 10-Q | 10-Q | 10-K | 10-Q | 10-Q | 10-Q | 10-K |
| Cash flows from operating activities: | | | | | | | | |
| Net loss | -41.5 | -191.3 | -20.3 | 34.0 | -12.5 | -46.3 | -30.1 | -7.8 |
| Adjustments to reconcile net loss to net cash from (used in) operating activities: | | | | | | | | |
| Depreciation, depletion and amortization | 26.7 | 26.4 | 26.0 | 6.9 | 38.1 | 39.5 | 36.6 | 72.2 |
| Share-based compensation | 0.0 | 0.0 | 1.8 | -0.6 | 1.4 | 1.1 | 1.3 | 1.7 |
| Amortization of deferred financing costs | 1.3 | 2.1 | 3.1 | 2.6 | 3.0 | 2.6 | 2.6 | 3.2 |
| Non-cash loss on extinguishment of debt | | 1.4 | | | | | | |
| Gain on derivative instruments | | | | 4.6 | -4.7 | 0.5 | -2.4 | -26.2 |
| (Gain) loss on foreign exchange | 1.1 | -0.3 | -1.3 | -0.3 | 1.7 | 1.1 | 0.5 | -0.8 |
| Income from equity affiliates | -1.0 | 0.1 | -1.2 | -1.6 | -1.4 | -1.4 | -1.5 | -1.5 |
| Changes in operating assets and liabilities: | | | | | | | | |
| Receivables | -11.6 | 16.6 | -2.8 | 36.9 | -12.6 | -0.9 | 12.3 | -14.2 |
| Inventories | 27.4 | -16.2 | -17.0 | 0.6 | 12.0 | 2.6 | 5.2 | 4.8 |
| Accounts payable and accrued expenses | 18.5 | -22.9 | 8.5 | 6.8 | -6.0 | 1.0 | -21.9 | 8.8 |
| Deferred revenue | -3.5 | -3.1 | 6.0 | -17.0 | -1.2 | -7.2 | 2.3 | -11.7 |
| Other assets and liabilities | 0.8 | -6.8 | -13.8 | 2.5 | 0.4 | 10.5 | 7.1 | -10.2 |
| Asset retirement obligation | -9.5 | -10.6 | -9.6 | | -12.2 | -10.2 | -10.7 | |
| Net cash provided by operating activities | 48.7 | -31.2 | -8.9 | 93.0 | 11.0 | 10.9 | -0.7 | 66.8 |
| |
| Cash flows from investing activities: | | | | | | | | |
| Additions to property, plant and equipment | -13.5 | -5.6 | -6.2 | -9.7 | -12.3 | -5.9 | -7.2 | -15.5 |
| Proceeds from sales of restricted investments | 6.2 | 13.3 | 10.3 | | 12.1 | 12.0 | 9.6 | |
| Proceeds from sales of assets | 1.1 | 2.0 | 0.1 | | 0.0 | 0.3 | 0.5 | |
| Net cash (used in) provided by investing activities | -12.5 | -2.2 | -7.5 | -6.0 | -14.5 | -5.5 | 36.0 | -8.3 |
| |
| Cash flows from financing activities: | | | | | | | | |
| Borrowings from long-term debt, net of debt discount | | | | | | | | |
| Repayments of long-term debt | -5.8 | -56.2 | -10.8 | -18.0 | -19.8 | -22.0 | -22.4 | -26.5 |
| Borrowings on revolving lines of credit | | 50.5 | 71.9 | 39.2 | 122.9 | -10.0 | 123.2 | 109.6 |
| Repayments of revolving lines of credit | -5.8 | -56.2 | -10.8 | -18.0 | -19.8 | -22.0 | -22.4 | -26.5 |
| Debt issuance costs and other refinancing costs | | -2.7 | | | | | | -1.5 |
| Net cash provided by (used in) financing activities | -5.8 | 26.8 | -9.8 | -28.7 | -9.4 | -22.1 | -22.5 | -28.2 |
| Net decrease in cash and cash equivalents, including restricted cash | -0.3 | | | | -16.6 | | | |
| |
| Supplemental disclosures of cash flow information: | | | | | | | | |
| Non-cash transactions: | | | | | | | | |
| Capital leases and other financing sources | 0.6 | | | 0.8 | 0.0 | | 0.5 | 7.5 |
| Cash and cash equivalents, including restricted cash, end of period | | | | | | | | |
| Cash and cash equivalents | | | | | | | | |
| Restricted cash in Restricted investments, reclamation deposits and bond collateral | | | | | | | | |