Business combinations - Additional Information (Detail) - USD ($) $ in Millions			3 Months Ended			4 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2015	Feb. 27, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Business Acquisition [Line Items]
Cash held in escrow	$ 3.9		$ 3.9			$ 3.9	$ 3.9
Accrued liabilities	51.3		51.3			51.3	51.3		$ 7.1
Business Combination, Acquisition and Financing Related Costs			0.4				14.8
Fair value of trade accounts receivable		$ 150.4
Non cash amortization charge in cost of sales								$ 0.4
Estimated fair value of trade payable	10.7		10.7			10.7	10.7
Goodwill	84.5		84.5			84.5	84.5
Revenues			368.7				506.5	0.1
Loss from continuing operations before income taxes			(13.5)		$ (2.6)		(38.7)	(4.7)
Real Alloy
Business Acquisition [Line Items]
Goodwill	84.5		84.5			84.5	84.5
Real Alloy Acquisition
Business Acquisition [Line Items]
Percentage of voting interests acquired		100.00%
Total purchase consideration		$ 496.2
Cash held in escrow		5.0
Payment for initial working capital adjustment			31.3
Estimated final working capital adjustment	2.4	2.4
Accrued liabilities	$ 2.2		2.2			2.2	2.2
Business Combination, Acquisition and Financing Related Costs			0.4	$ 14.4					$ 3.4
Inventories		173.9
Fair value adjustment to inventory		10.7					6.5
Non cash amortization charge in cost of sales			3.5				$ 7.2
Estimated useful life							20 years
Fair value assumptions, discount rate							13.50%
Estimated economic useful life							20 years
Decrease in the estimated fair value of identifiable intangible assets			0.6
Goodwill		$ 84.5
Revenues			368.7			506.4	$ 506.5	0.1
Loss from continuing operations before income taxes			$ 13.5		$ 2.6	$ 20.8	$ 38.7	$ 4.7
Real Alloy Acquisition | Minimum
Business Acquisition [Line Items]
Fair value assumptions, discount rate							6.70%
Real Alloy Acquisition | Maximum
Business Acquisition [Line Items]
Fair value assumptions, discount rate							13.20%