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Stock-Based Compensation (Tables)
3 Months Ended
Mar. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Four quarterly assumptions for the fair value of the right (option) to purchase shares under ESPP
The fair value of the right (option) to purchase shares under the ESPP is estimated on the date of grant (January 1, 2013) associated with the four quarterly purchase dates using the following assumptions:
 
2012
 
2013
Dividend yield
1.7
%
 
0.6
%
Expected volatility
32
%
 
31
%
Risk-free interest rate
0.02%, 0.06%, 0.09%, 0.12%

 
0.08%, 0.12%, 0.135%, 0.15%

Expected life (years)
0.25, 0.50, 0.75, 1.00

 
0.25, 0.50, 0.75, 1.00