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Pension and other postretirement benefits - Weighted average assumptions used to determine net periodic benefit cost (Details)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Rate of compensation increase, year one through three 3.00%    
Rate of compensation increase, year four and thereafter 4.00%    
Rate of compensation increase, year one through four   3.00% 3.00%
Rate of compensation increase, year five and thereafter   4.00% 4.00%
Rate of compensation increase, year one through five     3.00%
Rate of compensation increase, year six and thereafter     4.00%
Defined Benefit Pension Plans [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Measurement date 12/31/2014 12/31/2013 12/31/2012
Fiscal year end 12/31/2015 12/31/2014 12/31/2013
Discount rate 4.05% 4.89% 4.00%
Rate of compensation increase, group 1 3.00% 3.00% 3.00%
Rate of compensation increase, group 2 4.00% 4.00% 4.00%
Expected return on plan assets 7.16% 7.25% 7.25%
Other Postretirement Benefit Plans [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Measurement date 12/31/2014 12/31/2013 12/31/2012
Fiscal year end 12/31/2015 12/31/2014 12/31/2013
Discount rate 4.00% 4.99% 4.01%
Rate of compensation increase, group 1 3.00% 3.00% 3.00%
Rate of compensation increase, group 2 4.00% 4.00% 4.00%
Expected return on plan assets 0.00% 0.00% 0.00%