| Quarterly Financial Information [Text Block] |
| | Selected Quarterly Financial Data (Unaudited) | The following tables present unaudited financial data of the Trust for each quarter of 201
7 and 2016 (in thousands, except per share amounts):
Ìý | Ìý | Ìý | | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | | Income | Ìý | $ | 38,321 | Ìý | Ìý | $ | 42,497 | Ìý | Ìý | $ | 27,358 | Ìý | Ìý | $ | 24,238 | Ìý | | Income before income taxes | Ìý | $ | 33,913 | Ìý | Ìý | $ | 39,091 | Ìý | Ìý | $ | 24,741 | Ìý | Ìý | $ | 22,114 | Ìý | | Net income | Ìý | $ | 18,361 | Ìý | Ìý | $ | 26,405 | Ìý | Ìý | $ | 16,711 | Ìý | Ìý | $ | 14,885 | Ìý | | Net income per Sub-share Certificate | Ìý | $ | 2.35 | Ìý | Ìý | $ | 3.36 | Ìý | Ìý | $ | 2.12 | Ìý | Ìý | $ | 1.88 | Ìý | Ìý Ìý | Ìý | Ìý | | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | Ìý | | Ìý | | Income | Ìý | $ | 17,573 | Ìý | Ìý | $ | 14,273 | Ìý | Ìý | $ | 16,196 | Ìý | Ìý | $ | 11,898 | Ìý | | Income before income taxes | Ìý | $ | 16,081 | Ìý | Ìý | $ | 13,100 | Ìý | Ìý | $ | 15,103 | Ìý | Ìý | $ | 10,802 | Ìý | | Net income | Ìý | $ | 10,907 | Ìý | Ìý | $ | 8,929 | Ìý | Ìý | $ | 10,123 | Ìý | Ìý | $ | 7,280 | Ìý | | Net income per Sub-share Certificate | Ìý | $ | 1.37 | Ìý | Ìý | $ | 1.12 | Ìý | Ìý | $ | 1.26 | Ìý | Ìý | $ | 0.90 | Ìý | Ìý
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