Annual report pursuant to Section 13 and 15(d)

Summary of Restricted Stock Activity (Detail)

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Summary of Restricted Stock Activity (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Number of Shares/Units      
Outstanding at January 1 1,384,390 1,254,691 764,078
Granted 653,229 424,436 683,921
Vested (489,033) (288,204) (190,589)
Canceled   (4,613)  
Forfeited (14,423) (1,920) (2,719)
Outstanding at December 31 1,534,163 1,384,390 1,254,691
Weighted Average Grant Price      
Weighted Average Grant Date Fair Value, Outstanding, Beginning $ 16.85 $ 14.60 $ 11.10
Weighted Average Grant Date Fair Value, Granted $ 21.70 $ 19.45 $ 17.94
Weighted Average Grant Date Fair Value, Vested $ 17.00 $ 10.84 $ 12.63
Weighted Average Grant Date Fair Value, Cancelled   $ 18.35  
Weighted Average Grant Date Fair Value, forfeited $ 18.58 $ 14.34 $ 11.03
Weighted Average Grant Date Fair Value Outstanding, Ending $ 18.85 $ 16.85 $ 14.60