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Retirement benefits (Details 2)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Pension benefits
     
Weighted-average asset allocation      
Equity securities (as a percent) 68.00% 71.00%  
Fixed income (as a percent) 32.00% 29.00%  
Total weighted-average asset allocation (as a percent) 100.00% 100.00%  
Asset category, Target      
Equity securities, Target (as a percent) 70.00% 70.00%  
Equity securities, target minimum range (as a percent) 65.00% 65.00%  
Equity securities, target maximum range (as a percent) 75.00% 75.00%  
Fixed income securities, Target (as a percent) 30.00% 30.00%  
Fixed income securities, target minimum range (as a percent) 25.00% 25.00%  
Fixed income securities, target maximum range (as a percent) 35.00% 35.00%  
Total target (as a percent) 100.00% 100.00%  
Benefit obligation      
Discount rate (as a percent) 5.19% 5.68% 6.50%
Rate of compensation increase (as a percent) 3.50% 3.50% 3.50%
Net periodic benefit cost (years ended)      
Discount rate (as a percent) 5.68% 6.50% 6.625%
Expected return on plan assets (as a percent) 8.00% 8.25% 8.25%
Rate of compensation increase (as a percent) 3.50% 3.50% 3.50%
Expected rate of return on plan assets for 2012 (as a percent) 7.75%    
Other benefits
     
Weighted-average asset allocation      
Equity securities (as a percent) 69.00% 70.00%  
Fixed income (as a percent) 31.00% 30.00%  
Total weighted-average asset allocation (as a percent) 100.00% 100.00%  
Asset category, Target      
Equity securities, Target (as a percent) 70.00% 70.00%  
Equity securities, target minimum range (as a percent) 65.00% 65.00%  
Equity securities, target maximum range (as a percent) 75.00% 75.00%  
Fixed income securities, Target (as a percent) 30.00% 30.00%  
Fixed income securities, target minimum range (as a percent) 25.00% 25.00%  
Fixed income securities, target maximum range (as a percent) 35.00% 35.00%  
Total target (as a percent) 100.00% 100.00%  
Benefit obligation      
Discount rate (as a percent) 4.90% 5.60% 6.50%
Net periodic benefit cost (years ended)      
Discount rate (as a percent) 5.60% 6.50% 6.50%
Expected return on plan assets (as a percent) 8.00% 8.25% 8.25%
Rate of compensation increase (as a percent)     3.50%
Assumed health care trend rates in next fiscal year and future years      
Assumed health care trend rate for medical in next fiscal year (as a percent) 8.50% 9.00%  
Assumed health care trend rate for grading down in 2019 and thereafter (as a percent) 5.00% 5.00%  
Assumed health care trend rate for dental in next fiscal year (as a percent) 5.00% 5.00%  
Assumed health care trend rate for vision in next fiscal year (as a percent) 4.00% 4.00%