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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Property, Plant and Equipment [Line Items]        
Number of shares sold     23,000,000  
Proceeds from sale of subsidiary stock     $ 25,000,000  
Ratio of remaining net proceed     0.25  
Short-term investment maturity period     Three months or less  
Securities owned $ 0   $ 0  
Weighted average number of common shares outstanding 80,252,203 81,440,848 78,991,192 73,498,034
Minimum [Member]
       
Property, Plant and Equipment [Line Items]        
Property and Equipment, estimated useful life     3 years  
Maximum [Member]
       
Property, Plant and Equipment [Line Items]        
Property and Equipment, estimated useful life     10 years