Annual report pursuant to Section 13 and 15(d)

Income Per Share - Anti-dilutive common stock equivalents (Details)

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Income Per Share - Anti-dilutive common stock equivalents (Details) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Anti-dilutive common stock equivalents      
Total anti-dilutive common stock equivalents 45,766 473,792 145,238
Restricted Stock Awards | Service-based conditions      
Anti-dilutive common stock equivalents      
Total anti-dilutive common stock equivalents 458 35,182 17,366
Restricted Stock Awards | Market-based conditions      
Anti-dilutive common stock equivalents      
Total anti-dilutive common stock equivalents   6,323  
Stock Options      
Anti-dilutive common stock equivalents      
Total anti-dilutive common stock equivalents 45,308 432,287 127,872