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Consolidated Balance Sheets (Parenthetical) (USD $)
Jun. 30, 2011
Dec. 31, 2010
Stockholders' equity    
Preferred stock, par value $ 0.01 $ 0.01
Preferred stock, shares authorized 10,000,000 10,000,000
Preferred stock, shares outstanding 0 0
Common stock, par value $ 0.01 $ 0.01
Common stock, shares authorized 150,000,000 150,000,000
Common stock, shares outstanding 56,615,262 57,875,269
Treasury stock, shares 25,436,991 24,179,176