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Stock-Based Compensation - Roll Forward of Restricted Stock Grants (Details) - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Roll-forward of outstanding restricted stock grants issued    
Outstanding shares, Beginning Balance 306,768  
Outstanding shares, Ending Balance 310,356 306,768
Restricted Stock Units (RSUs)    
Roll-forward of outstanding restricted stock grants issued    
Outstanding shares, Beginning Balance 335,768 312,099
Weighted-Average Grant Date Price per Share, Beginning of Period $ 79.79 $ 74.17
Grants issued 82,050 60,613
Weighted-Average Grant Date Price Per Share, Grants issued $ 169.50 $ 124.05
Grants vested (84,962) (28,731)
Weighted-Average Grant Date Price Per Share, Grants vested $ 73.47 $ 63.47
Grants forfeited (3,500) (8,213)
Weighted-Average Grant Date Price per Share, Grants forfeited $ 77.81 $ 77.23
Outstanding shares, Ending Balance 329,356 335,768
Weighted-Average Grant Date Price per Share, End of Period $ 108.46 $ 79.79