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Quarterly Financial Data (Summarized (Unaudited) Quarterly Financial Information) (Detail) (USD $)
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3 Months Ended | 12 Months Ended | |||||||||
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Dec. 31, 2013
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Sep. 30, 2013
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Jun. 30, 2013
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Mar. 31, 2013
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Dec. 31, 2012
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Sep. 30, 2012
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Jun. 30, 2012
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Mar. 31, 2012
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Dec. 31, 2013
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Dec. 31, 2012
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Dec. 31, 2011
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| Quarterly Financial Information [Line Items] | |||||||||||
| Total Income (Loss) | $ 3,761,843 | $ 2,797,327 | $ (6,924,190) | $ 3,872,304 | $ (2,211,546) | $ 18,217,956 | $ (16,246,333) | $ 15,220,113 | $ 3,507,284 | $ 14,980,190 | $ 14,915,851 |
| Total Expenses | 146,576 | 165,565 | 183,346 | 188,826 | 216,543 | 215,972 | 246,651 | 263,318 | 684,313 | 942,484 | 1,047,912 |
| Expense Waivers | (16,030) | (30,338) | (46,652) | (44,756) | (64,418) | (37,603) | (19,969) | (17,456) | (137,776) | (139,446) | (104,053) |
| Net Expenses | 130,546 | 135,227 | 136,694 | 144,070 | 152,125 | 178,369 | 226,682 | 245,862 | 546,537 | 803,038 | 943,859 |
| Net Income (Loss) | $ 3,631,297 | $ 2,662,100 | $ (7,060,884) | $ 3,728,234 | $ (2,363,671) | $ 18,039,587 | $ (16,473,015) | $ 14,974,251 | $ 2,960,747 | $ 14,177,152 | $ 13,971,992 |
| Net Income (Loss) per Share | $ 3.70 | $ 1.87 | $ (7.25) | $ 3.18 | $ (2.34) | $ 12.17 | $ (8.33) | $ 8.52 | $ 1.50 | $ 10.02 | $ 6.31 |