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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Rate of compensation increase, year one through two 3.00%    
Rate of compensation increase, year three and thereafter   4.00%  
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%
Rate of compensation increase, year five and thereafter     4.00%
Defined Benefit Pension Plans [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Measurement date 12/31/2016 12/31/2015 12/31/2014
Fiscal year end 12/31/2017 12/31/2016 12/31/2015
Discount rate 4.15% 4.44% 4.05%
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 6.82% 7.10% 7.16%
Other Postretirement Benefit Plans [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Measurement date 12/31/2016 12/31/2015 12/31/2014
Fiscal year end 12/31/2017 12/31/2016 12/31/2015
Discount rate 4.05% 4.50% 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 0.00% 0.00% 0.00%