| Quarterly Financial Data | Unaudited quarterly financial data are as follows (in thousands, except per share amounts):  
|   | 1st Quarter |   |   | 2nd Quarter |   |   | 3rd Quarter |   |   | 4th Quarter |   |  
| Year Ended December 31, 2018 |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |  
| Revenues | $ | 134,213 |   |   | $ | 149,612 |   |   | $ | 153,459 |   |   | $ | 143,422 |   |  
| Operating  income |   | 38,739 |   |   |   | 48,467 |   |   |   | 57,147 |   |   |   | 102,674 |   |  
| Net income (1) |   | 27,640 |   |   |   | 36,083 |   |   |   | 46,260 |   |   |   | 138,844 |   |  
| Basic and diluted earnings per common share |   | 0.19 |   |   |   | 0.25 |   |   |   | 0.32 |   |   |   | 0.96 |   |  
|   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |  
| Year Ended December 31, 2017 |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |  
| Revenues | $ | 124,393 |   |   | $ | 123,323 |   |   | $ | 110,281 |   |   | $ | 129,099 |   |  
| Operating  income |   | 28,196 |   |   |   | 32,888 |   |   |   | 15,700 |   |   |   | 33,166 |   |  
| Net income (loss) (1) |   | 24,299 |   |   |   | 33,315 |   |   |   | (1,297 | ) |   |   | 23,365 |   |  
| Basic and diluted earnings (loss) per common share |   | 0.17 |   |   |   | 0.23 |   |   |   | (0.01 | ) |   |   | 0.16 |   |  
|   | (1) | During the fourth quarter of 2018, we recorded a gain on debt transactions of $47.1 million and a derivative gain of $59.7 million.  During the first quarter of 2017, we recorded a gain on debt transactions of $7.8 million.  See Note 2 and Note 9 for additional information.  |   |  
|   | (2) | The sum of the individual quarterly earnings (loss) per common share may not agree with the yearly amount due to each quarterly calculation is based on income for that quarter and the weighted average common shares outstanding for that quarter. |   |  |