Annual report pursuant to Section 13 and 15(d)

Note 11 - Selected Quarterly Financial Data (Unaudited) (Detail) - The following tables present unaudited financial data of the Trust for each quarter:

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Note 11 - Selected Quarterly Financial Data (Unaudited) (Detail) - The following tables present unaudited financial data of the Trust for each quarter: (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Income $ 32,587,456 $ 34,300,508 $ 20,065,965
Income before income taxes 29,322,886 30,755,918 16,424,181
Net income 19,647,818 20,594,769 11,308,711
Net income per Sub-share Certificate (in Dollars per share) $ 2.20 $ 2.21 $ 1.17
December 31 [Member]
      
Income 7,276,793 11,239,562  
Income before income taxes 6,218,508 10,009,760  
Net income 4,113,797 6,560,335  
Net income per Sub-share Certificate (in Dollars per share) $ 0.47 $ 0.71  
September 30 [Member]
      
Income 7,043,281 8,560,379  
Income before income taxes 6,369,983 7,848,114  
Net income 4,311,092 5,258,936  
Net income per Sub-share Certificate (in Dollars per share) $ 0.48 $ 0.56  
June 30 [Member]
      
Income 8,706,738 8,798,987  
Income before income taxes 7,956,109 8,012,551  
Net income 5,284,674 5,415,929  
Net income per Sub-share Certificate (in Dollars per share) $ 0.59 $ 0.58  
March 31 [Member]
      
Income 9,580,079 5,720,108  
Income before income taxes 8,778,286 4,885,493  
Net income $ 5,938,255 $ 3,359,569  
Net income per Sub-share Certificate (in Dollars per share) $ 0.65 $ 0.35