Schedule of Losses on Derivative Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	$ 0.8	$ 0.8
Unrealized Losses on derivative financial instruments	0.3	0.4
Losses on derivative financial instruments	1.1	1.2
Metal
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	0.8	0.2
Unrealized Losses on derivative financial instruments	0.0	0.4
Natural Gas
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	0.0	0.6
Unrealized Losses on derivative financial instruments	$ 0.3	$ 0.0