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Purchase commitments (Details 4) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Purchase commitments due      
2017 $ 367.7    
2018 215.7    
2019 189.4    
2020 138.0    
2021 130.6    
Thereafter 859.5    
Amounts purchased under various commitments $ 539.3 $ 842.1 $ 759.0
Minimum [Member]      
Long-term Purchase Commitment [Line Items]      
Term of unrecorded unconditional purchase obligation 1 year    
Maximum [Member]      
Long-term Purchase Commitment [Line Items]      
Term of unrecorded unconditional purchase obligation 44 years