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Stock Option Plans - Narrative (Details)
3 Months Ended 12 Months Ended
Mar. 31, 2016
USD ($)
compensation_plan
$ / shares
shares
Dec. 31, 2007
shares
Dec. 31, 2015
USD ($)
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Number of stock incentive plans | compensation_plan 2    
Option expiration period 10 years    
Option vesting period 4 years    
Closing share price (in usd per share) | $ / shares $ 53.02    
Options granted (in shares) | shares 0 70,903  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Intrinsic value of options $ 5,460,449   $ 4,237,456
Weighted average remaining contractual life of the Company's outstanding options 6 years 10 months 13 days    
Weighted average remaining contractual life of the Company's exercisable options 5 years 5 months 20 days    
Options Issued in 2007      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Intrinsic value of options $ 0