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Compensation Plans - Roll-Forward of Outstanding Restricted Stock Grants Issued (Detail) (Restricted Stock Units (RSUs) [Member], USD $)
9 Months Ended 12 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Restricted Stock Units (RSUs) [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Outstanding shares, Beginning Balance 319,988 260,621
Grants issued 31,000 107,600
Grants vested (18,649) (46,056)
Grants forfeited   (2,177)
Outstanding shares, Ending Balance 332,339 319,988
Weighted-Average Grant Date Price per Share, Beginning of Period $ 74.22 $ 73.78
Weighted-Average Grant Date Price per Share, Grants issued $ 84.57 $ 75.64
Weighted-Average Grant Date Price per Share, Grants vested $ 84.36 $ 75.22
Weighted-Average Grant Date Price per Share, Grants forfeited   $ 71.37
Weighted-Average Grant Date Price per Share, End of Period $ 74.61 $ 74.22