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Compensation Plans - Roll-Forward of Outstanding Restricted Stock Grants Issued (Detail) (USD $)
9 Months Ended 12 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Restricted Stock Units (RSUs)
Dec. 31, 2013
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Outstanding shares, Beginning Balance 312,099   312,099 319,988
Grants issued 33,745 59,006 41,013 32,000
Grants vested     (18,187) (39,749)
Grants forfeited     (7,813) (140)
Outstanding shares, Ending Balance 313,112   327,112 312,099
Weighted-Average Grant Date Price per Share, Beginning of Period     $ 74.17 $ 74.22
Weighted-Average Grant Date Price per Share, Grants issued     $ 120.27 $ 85.63
Weighted-Average Grant Date Price per Share, Grants vested     $ 61.08 $ 80.77
Weighted-Average Grant Date Price per Share, Grants forfeited     $ 78.00 $ 77.81
Weighted-Average Grant Date Price per Share, End of Period     $ 80.95 $ 74.17