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Inventories (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Inventories      
LIFO inventories - cost on the FIFO method $ 1,390.0 $ 1,219.0  
LIFO inventory valuation reserve adjustment (21.8) 51.5  
Lower of cost or market adjustment   (42.6)  
Cost on FIFO method (higher) lower than LIFO value (21.8) 8.9  
Inventories - stated on LIFO method 1,368.2 1,227.9  
Inventories - stated on FIFO method 357.8 304.7  
Inventories 1,726.0 1,532.6  
Lower of cost or market charge related to certain inventories of a foreign subsidiary   7.6  
LIFO inventory valuation reserve adjustment charge (income) $ 73.3 $ (8.5) $ (186.1)
Liquidation of LIFO inventory quantities that increased cost of sales     $ 38.7