Annual report pursuant to Section 13 and 15(d)

Inventories - Additional Information (Detail)

v3.10.0.1
Inventories - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Inventory Disclosure [Abstract]                      
Lower of cost or market adjustment to inventory $ 2.8 $ 1.2 $ (0.2) $ (0.9) $ (2.8) $ (0.7) $ 0.7 $ (0.8) $ 2.9 $ (3.6) $ (1.7)
Effect of LIFO inventory liquidation on income (0.1)       (1.0)       (0.1) (1.0) $ (1.9)
Inventory [Line Items]                      
Inventories at period end 218.2       208.1       218.2 208.1  
Excess of market over reported value 78.3       121.0       78.3 121.0  
Estimate of Fair Value Measurement [Member]                      
Inventory [Line Items]                      
Inventories at period end $ 296.5       $ 329.1       $ 296.5 $ 329.1