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Restricted Stock - Market Based Awards Fair Value Assumptions (Details) - Restricted Stock - Market Based Awards - $ / shares
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award    
Expected term of award 3 years 3 years
Risk-free interest rate 1.40% 1.20%
Expected Volatility 51.40% 52.30%
Granted $ 94.02 $ 72.54