Schedule of Losses on Derivative Financial Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	$ 1.2	$ 3.4
Unrealized Losses (Gains) on derivative financial instruments	(1.0)	0.8	$ 0.0
Losses on derivative financial instruments	0.2	4.2	$ 0.0
Metal
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	0.6	2.9
Unrealized Losses (Gains) on derivative financial instruments	0.3	0.0
Natural Gas
Derivative Instruments Gain Loss [Line Items]
Realized Losses on derivative financial instruments	0.6	0.5
Unrealized Losses (Gains) on derivative financial instruments	$ (1.3)	$ 0.8