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Equity Compensation Plans - Grant Date Fair Value (Details) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Employee Consultants and Directors Stock Options [Member]      
Stock Option Disclosures      
Weighted average estimated grant date fair value per share $ 15.05 $ 8.41 $ 3.84
Restricted and Perfomance Stock Units [Member]      
Stock Option Disclosures      
Weighted average estimated grant date fair value per share $ 23.34 $ 26.24 $ 8.95