Schedule of Activity Associated with Goodwill (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill And Other Intangible Assets [Line Items]
Balance at beginning of period		$ 104.3
Acquisition of Real Alloy			$ 104.6
Impairment	$ (61.8)	(61.8)	0.0	$ 0.0
Currency translation adjustments		(0.3)	(0.3)
Balance at end of period	42.2	42.2	104.3
RANA
Goodwill And Other Intangible Assets [Line Items]
Balance at beginning of period		95.4
Acquisition of Real Alloy			95.4
Impairment		(61.8)
Balance at end of period	33.6	33.6	95.4
RAEU
Goodwill And Other Intangible Assets [Line Items]
Balance at beginning of period		8.9
Acquisition of Real Alloy			9.2
Currency translation adjustments		(0.3)	(0.3)
Balance at end of period	$ 8.6	$ 8.6	$ 8.9