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Restricted Stock - Market Based Awards Fair Value Assumptions (Details) - Restricted Stock - Market Based Awards - $ / shares
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award    
Expected term of award 3 years 3 years
Risk-free interest rate 2.40% 1.40%
Expected Volatility 42.30% 51.40%
Granted $ 69.98 $ 94.02