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Stockholders' Equity - Summary of Anti-Dilutive Options Excluded (Details) - $ / shares
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Stockholders' Equity Note [Abstract]    
Options to purchase shares of common stock (in shares) 1,616,963 3,204,951
Weighted average exercise price of options (in dollars per share) $ 55.02 $ 50.83