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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]        
Outstanding, beginning balance (in shares) 366,770      
Granted (in shares) 106,764      
Converted (in shares) (90,385)      
Canceled (in shares) (4,888)      
Outstanding, ending balance (in shares) 378,261 366,770    
Unvested-December 31, 2017 (in shares)       378,261
Weighted-Average Grant-Date Fair Value, Outstanding, beginning balance (in dollars per share) $ 79.72      
Weighted-Average Grant-Date Fair Value, Granted (in dollars per share) 127.29 $ 81.86 $ 95.25  
Weighted-Average Grant-Date Fair Value, Converted (in dollars per share) 66.18      
Weighted-Average Grant-Date Fair Value, Canceled (in dollars per share) 90.54      
Weighted-Average Grant-Date Fair Value, Outstanding, ending balance (in dollars per share) 96.23 79.72    
Weighted-Average Grant-Date Fair Value, Unvested-December 31, 2017 (in dollars per share) $ 79.72 $ 79.72   $ 96.23
Weighted-Average Remaining Contractual Life, Outstanding-December 31, 2017 7 years 10 months 6 days      
Weighted-Average Remaining Contractual Life, Unvested-December 31, 2017 7 years 10 months 6 days      
Aggregate Intrinsic Value, Outstanding-December 31, 2017       $ 80,997
Aggregate Intrinsic Value, Unvested-December 31, 2017       $ 80,997