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Stockholder's Equity (Details 1) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Summary of shares repurchased under stock purchase program      
Number of shares repurchased 6,787 302,149 16,500
Average price of shares repurchased $ 37.70 $ 35.74 $ 36.69
Shares remaining to be repurchased 234,964 241,751 543,900