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Stock Plans (Details) (USD $)
In Millions, except Share data
3 Months Ended9 Months Ended3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2010
Restricted Stock Awards for Certain Management
Sep. 30, 2011
Restricted Stock Awards for Certain Management
Sep. 30, 2010
Restricted Stock Awards for Certain Management
Sep. 30, 2011
Restricted Stock Awards
Dec. 31, 2010
Restricted Stock Awards
Stock Plans (Textual) [Abstract]         
Restricted stock award granted by company     67,400 207,090 
Grant-date fair value for restricted stock award weighted-average       $ 33.36 
Vesting period from the date of grant     Vest in 2014   
Restricted stock award, outstanding       636,305471,894
Additional Compensation Cost Due to Modifications    $ 2.5 $ 2.5  
Compensation Cost$ 2.6$ 4.4$ 7.4$ 8.0