Business combinations - Summary of Selected Unaudited Pro forma Results of Operations (Detail) - USD ($) $ in Millions	3 Months Ended		4 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014
Total revenues:
Total revenues, As reported	$ 368.7			$ 506.5	$ 0.1
Loss from continuing operations before income taxes:
Loss from continuing operations before income taxes, As reported	13.5	$ 2.6		38.7	4.7
Real Alloy Acquisition
Total revenues:
Total revenues, As reported	368.7		$ 506.4	506.5	0.1
Total revenues, Pro forma	368.7	394.5		743.3	773.0
Loss from continuing operations before income taxes:
Loss from continuing operations before income taxes, As reported	(13.5)	(2.6)	$ (20.8)	(38.7)	(4.7)
Loss from continuing operations before income taxes, Pro forma	$ (12.9)	$ (8.2)		$ (27.4)	$ (35.3)