Schedule of Losses (Gains) on Derivative Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	$ 0.4	$ 0.8	$ 1.2	$ 0.7
Unrealized Losses (Gains) on derivative financial instruments	(1.9)	1.3	(1.5)	1.3
Losses (gains) on derivative financial instruments	(1.5)	2.1	(0.3)	2.0
Metal
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	0.2	0.8	0.4	0.7
Unrealized Losses (Gains) on derivative financial instruments	(0.8)	$ 1.3	(0.4)	$ 1.3
Natural Gas
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	0.2		0.8
Unrealized Losses (Gains) on derivative financial instruments	$ (1.1)		$ (1.1)