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Purchase commitments (Details 4) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Commitments and Contingencies Disclosure [Abstract]      
Term of unrecorded unconditional purchase obligation one to 45 years    
Purchase commitments due      
2016 $ 443.7    
2017 228.0    
2018 138.9    
2019 112.9    
2020 90.4    
Thereafter 853.9    
Amounts purchased under various commitments $ 861.4 $ 925.2 $ 860.5