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Employee Stock and Incentive Plans - Performance Shares and Restricted Stock Units (Details) (USD $)
In Millions, 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
Performance Shares [Member]
          
Number of Shares [Abstract]          
As of January 1, 107,899 128,333 128,333 122,489 121,825
Granted 41,332 33,525 38,764 [1] 39,312 [1] 49,302 [1]
Awarded 18,605   (41,009) (32,368) 0
Unearned Grant Award     (17,575) 0 (22,909)
Forfeited     (614) (1,100) (25,729)
As of December 31,     107,899 128,333 122,489
Weighted Average Grant Date Fair Value [Abstract]          
As of January 1, $ 40.73 $ 36.54 $ 36.54 $ 38.15 $ 41.96
Granted     $ 44.70 [1] $ 41.00 [1] $ 35.44 [1]
Awarded     $ 34.25 $ 48.10 $ 0.00
Unearned Grant Award     $ 34.25 $ 0.00 $ 54.50
Forfeited     $ 34.49 $ 34.35 $ 36.45
As of December 31,     $ 40.73 $ 36.54 $ 38.15
Performance Period     3 years    
Grants, Grant Date Fair Value $ 2.2 $ 1.5      
Awards, Grant Date Fair Value 0.7   1.4    
Restricted Stock Units [Member]
          
Number of Shares [Abstract]          
As of January 1, 56,415 63,464 63,464 43,803 28,983
Granted 19,193 16,355 18,162 [1] 20,136 [1] 26,589 [1]
Awarded 20,939   (24,707) (215) (3,091)
Forfeited     (504) (260) (8,678)
As of December 31,     56,415 63,464 43,803
Weighted Average Grant Date Fair Value [Abstract]          
As of January 1, $ 36.61 $ 32.57 $ 32.57 $ 30.61 $ 29.41
Granted     $ 40.83 [1] $ 36.74 [1] $ 31.83 [1]
Awarded     $ 29.43 $ 30.30 $ 29.75
Forfeited     $ 31.80 $ 29.41 $ 30.62
As of December 31,     $ 36.61 $ 32.57 $ 30.61
Performance Period     3 years    
Grants, Grant Date Fair Value 0.8 0.7      
Awards, Grant Date Fair Value $ 0.7   $ 0.7    
[1] Shares granted includes accrued dividends.