Inventories - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Inventory [Line Items]
Percentage of metal component in inventory	67.00%		67.00%		70.00%
Adjustment decrease to coat of sale	$ 0.7	$ 2.2	$ 0.8	$ 2.1
Inventories at period end	178.0		178.0		$ 163.7
Excess of market over reported value	95.0		95.0		69.2
Estimate of Fair Value Measurement [Member]
Inventory [Line Items]
Inventories at period end	$ 273.0		$ 273.0		$ 232.9