Business combinations - Summary of Selected Unaudited Pro forma Results of Operations (Detail) - USD ($) $ in Millions	2 Months Ended	3 Months Ended								10 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Total revenues:
Total revenues, As reported		$ 304.5	$ 314.9	$ 320.9	$ 309.4	$ 300.5	$ 338.6	$ 368.7	$ 137.8		$ 1,249.7	$ 1,145.6	$ 2.1
Loss from continuing operations before income taxes:
Loss from continuing operations before income taxes, As reported											(103.8)	(40.8)	(8.5)
Beck Aluminum International, LLC (“Beck Trading”)
Total revenues:
Total revenues, As reported	$ 10.5										1,249.7	1,145.6
Total revenues, Pro forma											1,326.7	1,285.7
Loss from continuing operations before income taxes:
Loss from continuing operations before income taxes, As reported	$ (1.1)										(103.2)	(31.7)
Loss from continuing operations before income taxes, Pro forma											$ (112.6)	(46.7)
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)