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Pension and other postretirement benefits - Expected Contribution Cash Flows (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Pension Plans [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Employer contributions $ 5,910 $ 5,584
Expected contributions in next twelve months 1,769  
Defined Benefit Plan, Expected Future Benefit Payments, Fiscal Year Maturity [Abstract]    
2016 26,438  
2017 19,755  
2018 20,228  
2019 21,229  
2020 21,655  
2021-2025 105,301  
Other Postretirement Benefit Plans [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Employer contributions 5,878 $ 4,681
Expected contributions in next twelve months 6,551  
Defined Benefit Plan, Expected Future Benefit Payments, Fiscal Year Maturity [Abstract]    
2016 6,551  
2017 6,789  
2018 7,099  
2019 7,388  
2020 7,508  
2021-2025 $ 39,031