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Information for Restricted Stock Units under the Equity Incentive Plan (Detail) (Restricted stock units, USD $)
6 Months Ended
Jun. 30, 2011
Restricted stock units
 
Shares  
Nonvested, beginning of period 395,950
Granted 312,735
Vested (112,528)
Forfeited (9,746)
Nonvested, end of period 586,411
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period $ 43.49
Granted $ 60.06
Vested $ 43.80
Forfeited $ 47.51
Nonvested, end of period $ 52.20