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Fair value measurements (Details) (USD $)
Mar. 31, 2013
Dec. 31, 2012
Recurring    
Marketable securities $ 8,572,723 $ 8,570,338
Nonrecurring    
Investment in affiliate 7,243,166 6,925,346
Fair Value, Inputs, Level 1 [Member]
   
Recurring    
Marketable securities 8,572,723 8,570,338
Nonrecurring    
Investment in affiliate 0 0
Fair Value, Inputs, Level 2 [Member]
   
Recurring    
Marketable securities 0 0
Nonrecurring    
Investment in affiliate 0 0
Fair Value, Inputs, Level 3 [Member]
   
Recurring    
Marketable securities 0 0
Nonrecurring    
Investment in affiliate $ 7,243,166 $ 6,925,346