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Stock Plans (Details) (USD $)
In Thousands, except Share data
3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Restricted Stock Awards for Certain Management
Jun. 30, 2011
Restricted Stock Awards
Dec. 31, 2010
Restricted Stock Awards
Stock Plans (Textuals) [Abstract]              
Restricted stock award granted by company         67,400 186,470  
Grant-date fair value for restricted stock award weighted-average           $ 34.02  
Vesting period from the date of grant         Vest in 2014    
Restricted stock award, outstanding           622,050 471,894
Compensation Cost $ 2,500 $ 1,900 $ 4,800 $ 3,534