Derivative and Other Financial Instruments and Fair Value Measurements - Schedule of Losses (Gains) on Derivative Financial Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments (Gain) Loss
Realized losses (gain) on derivative financial instruments	$ 1.9	$ 1.2	$ 3.4
Unrealized losses (gains) on derivative financial instruments	0.2	(1.0)	0.8
Losses on derivative financial instruments, net	2.1	0.2	4.2
Metal
Derivative Instruments (Gain) Loss
Realized losses (gain) on derivative financial instruments	1.8	0.6	2.9
Unrealized losses (gains) on derivative financial instruments	(0.4)	0.3
Natural Gas
Derivative Instruments (Gain) Loss
Realized losses (gain) on derivative financial instruments	0.1	0.6	0.5
Unrealized losses (gains) on derivative financial instruments	$ 0.6	$ (1.3)	$ 0.8