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Pension and other postretirement benefits - Funded status of plans (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Current liabilities $ (11,004) $ (10,917)
Defined Benefit Pension Plans [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Funded status (40,741) (51,306)
Current liabilities (1,812) (1,813)
Non-current liabilities (38,929) (49,493)
Net amount recognized (40,741) (51,306)
Net loss 71,199 83,451
Prior service cost (benefit) 998 1,104
Total 72,197 84,555
Projected benefit obligation for plans with accumulated benefit obligations in excess of plan assets 344,108 328,047
Accumulated benefit obligation for plans with accumulated benefit obligations in excess of plan assets 339,381 322,356
Fair value of plan assets for plans with accumulated benefit obligations in excess of plan assets 303,367 276,741
Other Postretirement Benefit Plans [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Funded status (120,764) (133,856)
Current liabilities (7,459) (7,501)
Non-current liabilities (113,305) (126,355)
Net amount recognized (120,764) (133,856)
Net loss 58,434 47,957
Prior service cost (benefit) (28,935) (6,595)
Total $ 29,499 $ 41,362