Schedule of Losses (Gains) on Derivative Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments Gain Loss [Line Items]
Realized losses (gain) on derivative financial instruments	$ 0.5	$ 0.4	$ 1.3	$ 1.2
Unrealized losses (gains) on derivative financial instruments	0.1	(1.9)	0.4	(1.5)
Losses (gains) on derivative financial instruments	0.6	(1.5)	1.7	(0.3)
Metal
Derivative Instruments Gain Loss [Line Items]
Realized losses (gain) on derivative financial instruments	0.6	0.2	1.4	0.4
Unrealized losses (gains) on derivative financial instruments	(0.2)	(0.8)	(0.2)	(0.4)
Natural Gas
Derivative Instruments Gain Loss [Line Items]
Realized losses (gain) on derivative financial instruments	(0.1)	0.2	(0.1)	0.8
Unrealized losses (gains) on derivative financial instruments	$ 0.3	$ (1.1)	$ 0.6	$ (1.1)