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Pension and other post-retirement benefits - Target Plan Asset Allocation (Details)
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 100.00%
Equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 69.00%
Debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 31.00%
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 22.00%
Maximum | Equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 78.00%
Maximum | Debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation percentage 51.00%