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Fair value measurements (Details) (USD $)
Sep. 30, 2012
Dec. 31, 2011
Recurring    
Cash equivalents $ 3,167,672 $ 16,177,462
Restricted cash 0 7,500,000
Marketable securities 8,561,807 8,496,372
Total Recurring 11,729,479 32,173,834
Nonrecurring    
Investment in affiliate 6,442,391 6,634,598
Fair Value, Inputs, Level 1 [Member]
   
Recurring    
Total Recurring 11,729,479 32,173,834
Nonrecurring    
Investment in affiliate 0 0
Fair Value, Inputs, Level 2 [Member]
   
Recurring    
Total Recurring 0 0
Nonrecurring    
Investment in affiliate 0 0
Fair Value, Inputs, Level 3 [Member]
   
Recurring    
Total Recurring 0 0
Nonrecurring    
Investment in affiliate 6,442,391 6,634,598
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
   
Recurring    
Cash equivalents 3,167,672 16,177,462
Restricted cash 7,500,000 7,500,000
Marketable securities 8,561,807 8,496,372
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
   
Recurring    
Cash equivalents 0 0
Restricted cash 0 0
Marketable securities 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
   
Recurring    
Cash equivalents 0 0
Restricted cash 0 0
Marketable securities 0 0
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member]
   
Nonrecurring    
Investment in affiliate 0 0
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member]
   
Nonrecurring    
Investment in affiliate 0 0
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
   
Nonrecurring    
Investment in affiliate $ 6,442,391 $ 6,634,598