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Summarized Status of Performance-based Nonvested Shares and Changes during Period (Detail) (Performance Shares, USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Performance Shares
     
Shares      
Outstanding at beginning of year 580,000 316,000 62,000
Granted 278,000 300,000 316,000
Vested (37,000) (36,000) (62,000)
Forfeited/canceled (12,000)    
Outstanding at end of year 809,000 580,000 316,000
Weighted Average Grant Date Fair Value      
Outstanding at beginning of year $ 16.68 $ 15.65 $ 13.69
Granted $ 22.91 $ 17.63 $ 15.65
Vested $ 18.08 $ 15.61 $ 13.69
Forfeited/canceled $ 16.57    
Outstanding at end of year $ 18.76 $ 16.68 $ 15.65