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Pension and other post-retirement benefits - Benefit Obligations in Excess of Plan Assets (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Accumulated benefit obligation $ 439,458 $ 462,943
Fair value of plan assets 339,099 376,276
Projected benefit obligation 476,791 523,743
Fair value of plan assets 339,099 403,945
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Accumulated benefit obligation 163,375 171,175
Fair value of plan assets 107,430 121,561
Projected benefit obligation 163,375 171,175
Fair value of plan assets $ 107,430 $ 121,561