Business combinations - Summary of Selected Unaudited Pro forma Results of Operations (Detail) - USD ($) $ in Millions	3 Months Ended						10 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Total revenues:
Total revenues, As reported	$ 300.5	$ 338.6	$ 368.7	$ 137.8	$ 2.0	$ 0.1		$ 1,145.6	$ 2.1	$ 5.3
Loss from continuing operations before income taxes:
Loss from continuing operations before income taxes, As reported								(40.8)	(8.5)	$ (16.5)
Real Alloy Acquisition
Total revenues:
Total revenues, As reported							$ 1,145.6	1,145.6	2.1
Total revenues, Pro forma								1,382.6	1,523.4
Loss from continuing operations before income taxes:
Loss from continuing operations before income taxes, As reported							$ 23.3	(40.8)	(8.5)
Loss from continuing operations before income taxes, Pro forma								$ (22.8)	$ (53.6)