| 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 |
| Operating activities: | | | | | | | | |
| Net income (loss) | -14.3 | -16.6 | | | -14.6 | 14.7 | | |
| Adjustments to reconcile net income (loss) to net cash from operating activities: | | | | | | | | |
| Depletion, depreciation and amortization | 33.0 | 23.5 | 23.5 | | 25.5 | 22.3 | 16.0 | |
| Amortization of deferred financing costs and long-term debt premium and discount | 0.7 | 0.6 | 0.5 | 0.5 | 0.5 | 0.5 | 7.7 | 1.3 |
| Stock-based compensation | 4.2 | 4.5 | 8.9 | 3.2 | 3.0 | 3.0 | 3.0 | 1.2 |
| Commodity derivative instruments (gain) loss | | | | | | | | |
| Commodity derivative settlement gain (loss) | | | | | | | | |
| Unrealized contingent payment derivative instrument gain | | | | | | | | |
| Change in operating assets and liabilities: | | | | | | | | |
| Accounts receivable | 4.5 | -0.3 | 7.4 | 4.9 | -1.2 | -21.9 | 10.5 | -8.0 |
| Other current assets | 0.5 | -0.6 | -0.3 | 0.1 | -0.5 | -0.1 | 0.0 | 0.5 |
| Accounts payable and accrued expenses | 12.6 | 1.5 | 7.9 | -6.0 | 20.1 | 8.0 | 3.2 | 16.1 |
| Accrued interest payable | 12.8 | -10.1 | 11.2 | -11.1 | 10.9 | -6.4 | 8.4 | -8.6 |
| Net cash provided by operating activities | 78.3 | 14.4 | 45.9 | 25.3 | 59.8 | 12.9 | 39.3 | 25.0 |
| |
| Investing activities: | | | | | | | | |
| Oil and gas exploration and development expenditures | -110.4 | -123.2 | -57.5 | -87.1 | -100.5 | -76.2 | -42.3 | -30.0 |
| Proceeds from sale of oil and gas properties | 0.5 | 5.3 | 2.5 | | 8.1 | 6.1 | 14.2 | |
| Purchase of oil and gas properties | | | | | | -161.3 | | |
| Deposit for Aneth disposition | | | | | 10.0 | | | |
| Purchase of other property and equipment | -1.2 | -1.3 | -0.8 | 0.1 | -1.1 | -2.4 | 0.0 | |
| Restricted cash | | | | 1.1 | 0.0 | 0.7 | -0.7 | |
| Other long-term assets | | | | | 0.0 | 0.0 | 0.0 | 0.0 |
| Net cash used in investing activities | -111.1 | -119.2 | -55.9 | 73.9 | -83.5 | -217.0 | -44.9 | -123.4 |
| |
| Financing activities: | | | | | | | | |
| Proceeds from bank borrowings | 102.0 | 170.0 | 67.0 | 90.0 | 78.0 | 140.0 | 73.0 | 83.0 |
| Repayments of bank borrowings | -65.0 | -137.0 | -57.0 | -185.0 | -53.0 | -59.0 | -64.0 | -73.0 |
| Proceeds from issuance of senior notes | | | | | | | | |
| Repayment of term loan | | | | | | | -128.3 | |
| Payment of financing costs | -0.3 | -0.7 | | 0.0 | -0.2 | -2.5 | -2.6 | |
| Payment of preferred dividend | -1.3 | -1.3 | -1.3 | -1.3 | -1.3 | -1.3 | -1.4 | |
| Redemption of restricted stock for employee income taxes | -0.2 | -0.8 | -2.1 | 0.0 | -0.1 | -0.1 | -3.2 | 0.0 |
| Proceeds from exercise of employee options to purchase common stock | 0.1 | 0.1 | 0.2 | 0.0 | 0.1 | 0.1 | 0.1 | 0.1 |
| Net cash provided by financing activities | 35.3 | 105.0 | 6.8 | -96.3 | 23.5 | 204.2 | -126.5 | 230.6 |
| Net decrease in cash and cash equivalents | 2.5 | 0.2 | -3.1 | 2.9 | -0.2 | 0.1 | -132.0 | 132.2 |
| Cash and cash equivalents at beginning of period | | | 3.8 | | | | 133.1 | |
| Cash and cash equivalents at end of period | 2.5 | 0.2 | 0.6 | 2.9 | -0.2 | 0.1 | 1.0 | 132.2 |