| In millions, except per share items | Dec-31-22 | Dec-31-21 | Dec-31-20 | Dec-31-19 | Dec-31-18 | Dec-31-17 | Dec-31-16 | Dec-31-15 |
| | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K |
| Cash flows from operating activities | | | | | | | | |
| Net loss | -8.9 | 60.4 | -20.3 | 35.5 | 35.5 | 27.9 | 97.7 | -30.1 |
| |
| Adjustments to reconcile net loss to net cash (used in) from operating activities: | | | | | | | | |
| Depreciation, amortization, depletion and accretion | 6.4 | 7.9 | 8.5 | 7.4 | 0.7 | 0.8 | 1.0 | 2.0 |
| Operating lease expense | 2.7 | 2.0 | 3.6 | 3.2 | | | | |
| Stock-based compensation expense | 2.0 | 1.9 | 2.5 | 2.0 | 2.5 | 2.2 | 2.9 | 6.9 |
| Earnings from equity method investments | -3.5 | -68.7 | -31.0 | -69.2 | -54.2 | -53.8 | -45.6 | -8.9 |
| Amortization of debt discount and debt issuance costs | | 0.9 | 1.4 | 1.7 | 0.1 | 0.1 | | |
| Other non-cash items, net | 0.5 | -0.2 | 0.2 | 0.6 | 0.3 | 0.5 | 0.0 | 0.3 |
| Changes in operating assets and liabilities: | | | | | | | | |
| Prepaid expenses and other assets | -0.9 | -2.1 | 3.2 | -2.2 | -0.8 | -0.4 | 1.2 | -0.1 |
| Other long-term assets, net | 0.2 | -4.3 | -1.0 | -0.3 | -0.8 | 0.0 | 1.0 | 0.2 |
| Other current liabilities | 1.0 | -8.3 | -0.5 | -0.3 | -0.9 | -1.6 | -3.1 | -0.8 |
| Operating lease liabilities | 1.5 | 3.3 | | | | | | |
| Other long-term liabilities | 0.0 | -2.6 | -3.3 | -0.3 | -0.2 | 0.2 | -2.2 | 0.6 |
| Distributions from equity method investees, return on investment | 2.3 | 22.9 | 62.4 | 73.9 | 5.5 | 4.6 | 7.9 | 5.0 |
| Net cash (used in) provided by operating activities | -6.1 | 26.0 | 54.0 | 62.3 | -9.9 | -11.7 | -18.3 | -29.9 |
| |
| Cash flows from investing activities | | | | | | | | |
| Acquisition of property, plant, equipment, and intangible assets, net | -8.9 | -6.2 | -6.7 | -7.9 | -0.5 | -0.4 | | |
| Acquisition of mine development costs | | | | -0.7 | -62.5 | | | -2.1 |
| Distributions from equity method investees in excess of cumulative earnings | 3.6 | 51.1 | | | 47.2 | 48.9 | 38.3 | 8.7 |
| Proceeds from sale of property and equipment | 1.3 | 0.9 | | | | | 0.1 | 0.9 |
| Net cash (used in) provided by investing activities | -4.6 | 44.4 | -7.5 | -13.2 | -16.5 | 48.4 | 39.9 | 4.3 |
| |
| Cash flows from financing activities | | | | | | | | |
| Net proceeds from common stock issuance | | | | | | | | |
| Principal payments on finance lease obligations | -1.2 | -1.2 | -1.4 | -1.4 | | | | |
| Principal payments on Arq Loan | | | | | | | | |
| Dividends paid on common stock | 0.0 | -0.1 | -5.0 | -18.3 | -20.2 | -15.7 | | |
| Net cash provided by (used) in financing activities | -1.7 | -17.5 | -27.7 | -55.7 | 19.5 | -32.9 | -15.7 | 10.0 |
| |
| Supplemental disclosure of non-cash investing and financing activities: | | | | | | | | |
| Change in accrued purchases for property and equipment | 0.5 | 0.2 | | | | | | |