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Summary of Movement in Non-Vested Share Options (Detail) (Stock Option, USD $)
3 Months Ended
Mar. 31, 2013
Stock Option
 
Number of Shares  
Beginning Balance 2,094,533
Granted 12,450
Vested (455,900)
Forfeited (101,966)
Ending Balance 1,549,117
Weighted Average Exercise Price  
Beginning balance $ 22.43
Granted $ 31.49
Vested $ 25.59
Forfeited $ 23.34
Ending balance $ 21.51
Weighted Average Fair Value  
Beginning Balance $ 9.17
Granted $ 11.76
Vested $ 9.90
Forfeited $ 8.95
Ending Balance $ 8.95