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Non-Vested Restricted Stock Activity (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Non-vested restricted stock  
Beginning Balance 577,609
Granted 296,017
Vested (164,011)
Forfeited (22,837)
Ending Balance 686,778
Non-vested restricted stock, Weighted Average Grant Date Fair Value  
Beginning Balance $ 30.30
Granted $ 32.62
Vested $ 29.23
Forfeited $ 31.86
Ending Balance $ 31.50