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Summarizes Information of Performance Share Awards (Detail) (USD $)
12 Months Ended
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Non-vested at December 31, 2012 328,100
Granted 210,500
Cancelled (39,100)
Non-vested at December 31, 2013 499,500
Non-vested at December 31, 2012 $ 19.47
Granted $ 34.40
Cancelled $ 24.38
Non-vested at December 31, 2013 $ 25.38