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Investments - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Schedule of Held-to-maturity Securities [Line Items]      
Investment income $ 2,451,731 $ 1,953,619 $ 179,069
Held-to-maturity securities 52,142,302 22,606,453  
Held-to-maturity Securities
     
Schedule of Held-to-maturity Securities [Line Items]      
Investment income 3,044,856 1,368,358 225,213
Interest receivable 539,586 527,771  
Held-to-maturity Securities | Debt Securities of Certain Real Estate Funds
     
Schedule of Held-to-maturity Securities [Line Items]      
Held-to-maturity securities $ 11,917,947