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Share-Based Compensation Plan (Restricted Stock Unit Activity) (Details) (Restricted Stock Units (RSUs) [Member], USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Restricted Stock Units (RSUs) [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Number of Shares [Roll Forward]      
Beginning balance, Number of Units 130,512 80,852 71,576
Granted, Number of Units 69,772 52,087 40,629
Canceled (2009 grant), Number of Units     (41,248)
Modified (2009 grant), Number of Units     41,248
Converted into shares of common stock, Number of Units (35,346) (2,427)  
Forfeited/expired, Number of Units (8,428) 0 (31,353)
Ending balance, Number of Units 156,510 130,512 80,852
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Weighted Average Grant Date Fair Value [Abstract]      
Beginning balance, Weighted Average Grant Date Fair Value Per Share (USD per share) $ 20.90 $ 22.94 $ 19.38
Granted, Weighted Average Grant Date Fair Value Per Unit (USD per Unit) $ 22.20 $ 17.87 $ 22.62
Canceled (2009 grant), Weighted Average Grant Date Fair Value Per Unit (USD per Unit)     $ 18.04
Modified (2009 grant), Weighted Average Grant Date Fair Value Per Unit (USD per Unit)     $ 23.25
Converted into shares of common stock, Weighted Average Grant Date Fair Value Per Unit (USD per Unit) $ 23.27 $ 23.70  
Forfeited/expired, Weighted Average Grant Date Fair Value Per Unit (USD per Unit) $ 23.05 $ 0.00 $ 21.26
Ending balance, Weighted Average Grant Date Fair Value Per Share (USD per share) $ 20.83 $ 20.90 $ 22.94