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Stock-Based Compensation (Summary of Stock Option Activity) (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2008
Dec. 31, 2007
Dec. 31, 2006
Stock-Based Compensation [Abstract]                  
Options, Outstanding, beginning of year 226,739                
Options, Granted 1,100                
Options, Exercised (6,270)                
Options, Forfeited (5,583)                
Options, Outstanding, end of year 215,986   226,739            
Options, Exercisable, end of year 184,636                
Weighted Average Exercise Price Per Share, Outstanding, beginning of year $ 26.27                
Weighted Average Exercise Price Per Share, Granted $ 27.55                
Weighted Average Exercise Price Per Share, Exercised $ 22.62                
Weighted Average Exercise Price Per Share, Forfeited $ 27.89                
Weighted Average Exercise Price Per Share, Outstanding, end of year $ 26.34   $ 26.27            
Weighted Average Exercise Price Per Share, Exercisable, end of year $ 26.22                
Weighted Average Remaining Contractual Term, Outstanding, 6 years   6 years 2 months 6 days            
Weighted Average Remaining Contractual Term, Granted in Period 9 years 9 months 13 days                
Weighted Average Remaining Contractual Term, Exercised During Period 2 years 11 months 25 days                
Weighted Average Remaining Contractual Term, Forfeited During Period 4 years 4 months 20 days                
Weighted Average Remaining Contractual Term, Outstanding, 6 years   6 years 2 months 6 days            
Weighted Average Remaining Contractual Term, Exercisable at End of Period 5 years 4 months 20 days                
Aggregate Intrinsic Value, Outstanding, $ 256                
Aggregate Intrinsic Value, Granted in Period 30                
Aggregate Intrinsic Value, Outstanding, end of year 347   256            
Aggregate Intrinsic Value, Exercisable, end of year 324                
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 1,000   30,250 31,900 30,800 29,700 26,400 24,200 52,470
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 12 months                
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 121                
Share Price $ 30.60 $ 26.50 $ 29.75            
Proceeds from Stock Options Exercised 142 39              
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value $ 347   $ 256