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Pension benefits (Details 3)
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Maximum [Member]
     
Discount rate 14.00% 9.50% 9.90%
Rate of increase in compensation levels 9.00% 9.00% 9.00%
Minimum [Member]
     
Discount rate 5.80% 7.20% 8.50%
Rate of increase in compensation levels 7.00% 7.00% 7.00%