Schedule of Losses on Derivative Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	$ 0.2	$ 1.5	$ 1.4	$ 2.2
Unrealized Losses (Gains) on derivative financial instruments	0.6	(0.5)	(0.9)	0.8
Losses on derivative financial instruments	0.8	1.0	0.5	3.0
Metal
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	0.2	1.5	0.6	2.2
Unrealized Losses (Gains) on derivative financial instruments	0.2	(0.8)	(0.2)	0.5
Natural Gas
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	0.0	0.0	0.8	0.0
Unrealized Losses (Gains) on derivative financial instruments	$ 0.4	$ 0.3	$ (0.7)	$ 0.3