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Summary of Vesting Activities for Investment Share Program and Restricted Stock Awards (Detail) (USD $)
12 Months Ended
Dec. 31, 2011
Number of shares  
Non-vested at beginning of period 173,226
Granted 30,672
Vested (54,288)
Forfeited (8,219)
Non-vested at end of period 141,391
Weighted Average Fair Value  
Non-vested at beginning of period $ 28.36
Granted $ 70.20
Vested $ 27.19
Forfeited $ 39.18
Non-vested at end of period $ 37.25