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Stock-Based Compensation (Summary of Restricted Stock Unit Activity) (Details) - Restricted Stock Units (RSUs) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Number of Shares            
Outstanding, beginning balance (in shares) 378,261 366,770 277,719      
Granted (in shares) 80,254 106,764 140,452      
Converted (in shares) (97,997) (90,385) (44,656)      
Canceled (in shares) (9,497) (4,888) (6,745)      
Outstanding, ending balance (in shares) 351,021 378,261 366,770      
Unvested (in shares)       351,021    
Weighted-Average Grant-Date Fair Value            
Weighted-Average Grant-Date Fair Value, Outstanding, beginning balance (in dollars per share) $ 96.23 $ 79.72 $ 77.22      
Weighted-Average Grant-Date Fair Value, Granted (in dollars per share) 227.45 127.29 81.86      
Weighted-Average Grant-Date Fair Value, Converted (in dollars per share) 91.62 66.18 70.64      
Weighted-Average Grant-Date Fair Value, Canceled (in dollars per share) 121.37 90.54 81.89      
Weighted-Average Grant-Date Fair Value, Outstanding, ending balance (in dollars per share) 126.93 96.23 79.72      
Weighted-Average Grant-Date Fair Value, Unvested (in dollars per share) $ 96.23 $ 79.72 $ 77.22 $ 126.93 $ 96.23 $ 79.72
Additional Disclosures            
Weighted-Average Remaining Contractual Life, Outstanding 2 years 7 months 13 days 2 years 6 months 18 days 2 years 6 months 18 days      
Weighted-Average Remaining Contractual Life, Unvested 2 years 7 months 13 days          
Aggregate Intrinsic Value, Outstanding       $ 39,767 $ 80,997 $ 36,204
Aggregate Intrinsic Value, Unvested       $ 39,767