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Stock-Based Compensation (Summary of Performance Stock Unit Activity) (Details) - Performance Shares - 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) 75,949 54,505 27,233      
Granted (in shares) 33,706 21,444 27,272      
Converted (in shares) 0 0 0      
Canceled (in shares) 0 0 0      
Outstanding, ending balance (in shares) 109,655 75,949 54,505      
Unvested (in shares)       109,655    
Weighted-Average Grant-Date Fair Value            
Weighted-Average Grant-Date Fair Value, Outstanding, beginning balance (in dollars per share) $ 119.45 $ 108.51 $ 128.54      
Weighted-Average Grant-Date Fair Value, Granted (in dollars per share) 238.12 147.25 88.51      
Weighted-Average Grant-Date Fair Value, Outstanding, ending balance (in dollars per share) 146.96 119.45 108.51      
Weighted-Average Grant-Date Fair Value, Unvested (in dollars per share) $ 119.45 $ 108.51 $ 128.54 $ 146.96 $ 119.45 $ 108.51
Additional Disclosures            
Weighted-Average Remaining Contractual Life, Outstanding 1 year 9 months 7 days 1 year 11 months 4 days 2 years 7 months 20 days      
Weighted-Average Remaining Contractual Life, Unvested 1 year 9 months 7 days          
Aggregate Intrinsic Value, Outstanding       $ 12,423 $ 16,263 $ 5,380
Aggregate Intrinsic Value, Unvested       $ 12,423