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Stock-Based Compensation Plans (Details) - USD ($)
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Stock based compensation plans:    
Stock-based compensation expense $ 5,316,837 $ 1,891,067
Tax benefits from stock-based compensation expense $ 1,948,089 $ 694,022
Stock options    
Stock based compensation plans:    
Grants in period, options 0 0
Restricted stock    
Stock based compensation plans:    
Grants in period, other than options 222,408 214,832
Performance and market-based stock    
Stock based compensation plans:    
Grants in period, other than options 91,612 69,213