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Stock-Based Compensation - Summary of 2009 ESPP (Details) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Weighted average per share discount from market value for shares purchased (in dollars per share) $ 13.10 $ 15.20 $ 13.00
The 2009 ESPP      
Shares purchased pursuant to 2009 ESPP (in shares) 143,227    
Weighted average per share price of shares purchased (in dollars per share) $ 74.24    
Weighted average per share discount from market value for shares purchased (in dollars per share) $ 13.10    
Shares available for purchase pursuant to 2009 ESPP (in shares) 138,643