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Stock-Based Compensation - Employee Stock Purchase Plan (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Jul. 31, 2017
Jan. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Common shares issued for ESPP (in shares)   50,141 155,673 75,974  
ESPP expense     $ 0.4 $ 0.3 $ 0.2
Treasury shares issued for ESPP (in shares) 105,532        
Employee Stock Purchase Plan | Common Stock          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Fair market value at grant purchase price (percentage)     85.00%    
Fair market value at grant exercise price (percentage)     85.00%    
Common stock, reserved for future issuance (in shares)     3,000,000    
Common stock, shares available for future issuance (in shares)     2,700,000