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Pension and other post-retirement benefits - Benefit Obligations in Excess of Plan Assets (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Accumulated benefit obligation $ 504,403 $ 439,458
Fair value of plan assets 407,074 339,099
Projected benefit obligation 564,971 476,791
Fair value of plan assets 407,074 339,099
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Accumulated benefit obligation 202,422 163,375
Fair value of plan assets 133,711 107,430
Projected benefit obligation 202,422 163,375
Fair value of plan assets $ 133,711 $ 107,430