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Schedule of Nonvested Deferred Stock Units (Detail) (Nonvested Deferred Stock Units, USD $)
12 Months Ended
Dec. 31, 2012
Nonvested Deferred Stock Units
 
Number of Shares  
Outstanding, Beginning 46,146
Granted 25,352
Vested (24,659)
Outstanding, Ending 46,839
Vested, Ending 90,092
Expected to vest, Ending 45,484
Weighted-Average Grant Date Fair Value Per Share  
Outstanding, Beginning $ 34.85
Granted $ 36.42
Vested $ 34.69
Nonvested outstanding, Ending $ 35.78
Vested, Ending $ 31.39
Expected to vest, Ending $ 36.85