Schedule of Realized Losses (Gains) on Derivative Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Derivative Instruments Gain Loss [Line Items]
Losses (Gains) on derivative financial instruments	$ 0.8	$ (0.1)
Metal
Derivative Instruments Gain Loss [Line Items]
Losses (Gains) on derivative financial instruments	0.2	$ (0.1)
Natural Gas
Derivative Instruments Gain Loss [Line Items]
Losses (Gains) on derivative financial instruments	$ 0.6