Business combinations - Summary of Selected Unaudited Pro forma Results of Operations (Detail) - USD ($) $ in Millions	3 Months Ended		7 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014
Total revenues:
Total revenues, As reported	$ 338.6			$ 845.1	$ 0.1
Earnings (loss) from continuing operations before income taxes:
Loss from continuing operations before income taxes, As reported	2.5	$ (1.4)		(36.2)	(6.1)
Real Alloy Acquisition
Total revenues:
Total revenues, As reported	338.6		$ 845.0	845.1	0.1
Total revenues, Pro forma	338.6	392.6		1,081.9	1,165.6
Earnings (loss) from continuing operations before income taxes:
Loss from continuing operations before income taxes, As reported	2.5	(1.4)	$ 20.4	(36.2)	(6.1)
Loss from continuing operations before income taxes, Pro forma	$ 3.0	$ (2.4)		$ (18.5)	$ (43.6)