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Stock-Based Compensation - Roll Forward of Restricted Stock Grants (Details) - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Roll-forward of outstanding restricted stock grants issued    
Outstanding shares, Beginning Balance 310,356  
Outstanding shares, Ending Balance 201,800 310,356
Restricted Stock Units (RSUs)    
Roll-forward of outstanding restricted stock grants issued    
Outstanding shares, Beginning Balance 329,356 335,768
Weighted-Average Grant Date Price per Share, Beginning of Period $ 108.46 $ 79.79
Grants issued 38,900 82,050
Weighted-Average Grant Date Price Per Share, Grants issued $ 183.14 $ 169.50
Grants vested (127,386) (84,962)
Weighted-Average Grant Date Price Per Share, Grants vested $ 83.50 $ 73.47
Grants forfeited (17,070) (3,500)
Weighted-Average Grant Date Price per Share, Grants forfeited $ 143.69 $ 77.81
Outstanding shares, Ending Balance 223,800 329,356
Weighted-Average Grant Date Price per Share, End of Period $ 132.96 $ 108.46