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Pension and other post-retirement benefits - Target Plan Asset Allocation (Details)
Dec. 31, 2017
Equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 70.00%
Debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 30.00%
Other  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 0.00%
Minimum | Equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 49.00%
Minimum | Debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 21.00%
Maximum | Equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 79.00%
Maximum | Debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 51.00%