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Stock-Based Compensation - Summary of 2009 ESPP (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Weighted average per share discount from market value for shares purchased (in dollars per share) $ 15.20 $ 13.00 $ 15.89
The 2009 ESPP      
Shares purchased pursuant to 2009 ESPP (in shares) 90,881    
Weighted average per share price of shares purchased (in dollars per share) $ 86.13    
Weighted average per share discount from market value for shares purchased (in dollars per share) $ 15.20    
Shares available for purchase pursuant to 2009 ESPP (in shares) 281,870