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Stock-Based Compensation (Weighted Average Grant Date Fair Values) (Details) - $ / shares
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Stock Option [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Weighted average grant-date fair value (in dollars per share) $ 9.92 $ 2.51
ESPP [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Weighted average grant-date fair value (in dollars per share) $ 3.71 $ 2.31