Note 12 - Selected Quarterly Financial Data (Unaudited) - Unaudited Financial Data of the Trust for Each Quarter (Details) - USD ($) $ / shares in Units, $ in Thousands |
3 Months Ended | 12 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2017 |
Sep. 30, 2017 |
Jun. 30, 2017 |
Mar. 31, 2017 |
Dec. 31, 2016 |
Sep. 30, 2016 |
Jun. 30, 2016 |
Mar. 31, 2016 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Income | $ 38,321 | $ 42,497 | $ 27,358 | $ 24,238 | $ 17,573 | $ 14,273 | $ 16,196 | $ 11,898 | $ 132,329 | $ 59,911 | $ 79,415 |
| Income before income taxes | 33,913 | 39,091 | 24,741 | 22,114 | 16,081 | 13,100 | 15,103 | 10,802 | 119,860 | 55,087 | 75,283 |
| Net income | $ 18,361 | $ 26,405 | $ 16,711 | $ 14,885 | $ 10,907 | $ 8,929 | $ 10,123 | $ 7,280 | $ 76,361 | $ 37,240 | $ 50,039 |
| Net income per Sub-share Certificate (in dollars per share) | $ 2.35 | $ 3.36 | $ 2.12 | $ 1.88 | $ 1.37 | $ 1.12 | $ 1.26 | $ 0.90 | $ 9.72 | $ 4.66 | $ 6.10 |
| X | ||||||||||
- Definition The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period. No definition available.
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| X | ||||||||||
- Definition Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
- Definition The portion of profit or loss for the period, net of income taxes, which is attributable to the parent. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
- Definition Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss). Reference 1: http://www.xbrl.org/2003/role/presentationRef
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