Business combinations - Additional Information (Detail) - USD ($) $ in Millions			3 Months Ended						10 Months Ended	12 Months Ended
	Sep. 03, 2015	Feb. 27, 2015	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
Business Acquisition [Line Items]
Cash held in escrow			$ 3.9						$ 3.9	$ 3.9
Business Combination, Acquisition and Financing Related Costs										14.8	$ 3.4
Goodwill			104.3						104.3	104.3
Increase in goodwill			19.6
Acquisition of Real Alloy										104.6
Fair value of trade accounts receivable		$ 150.4
Amortization of purchase accounting adjustments										9.2
Environmental liabilities			4.3						4.3	4.3
Business acquisition, goodwill, expected tax deductible amount			66.3						66.3	66.3
Total deferred tax assets			388.2				$ 390.7		388.2	388.2	390.7
Total deferred tax liabilities			12.5				0.2		12.5	12.5	0.2
Revenues			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										(40.8)	(8.5)	$ (16.5)
Real Alloy
Business Acquisition [Line Items]
Goodwill			104.6						104.6	104.6
RANA
Business Acquisition [Line Items]
Goodwill			95.4						95.4	95.4
Acquisition of Real Alloy										95.4
Revenues										711.4
RAEU
Business Acquisition [Line Items]
Goodwill			8.9						8.9	8.9
Acquisition of Real Alloy										9.2
Total deferred tax assets			0.7						0.7	0.7
Total deferred tax liabilities			7.5						7.5	7.5
Revenues										$ 434.2
Transition Service Agreement
Business Acquisition [Line Items]
Service period										24 months
Services expense										$ 7.4
Real Alloy Acquisition
Business Acquisition [Line Items]
Percentage of voting interests acquired		100.00%
Cash held in escrow		$ 5.0
Payment for initial working capital adjustment from seller					$ 31.3
Payment for final working capital adjustment from seller	$ 2.4
Business Combination, Acquisition and Financing Related Costs										14.8	3.4
Goodwill		104.6	104.6						104.6	104.6
Acquisition of Real Alloy										102.3
Inventories		157.7	6.4						6.4	$ 6.4
Fair value adjustment to inventory		4.2
Fair value assumptions, discount rate										13.50%
Estimated economic useful life										20 years
Amortization of purchase accounting adjustments										$ 5.6
Business acquisition purchase price allocation adjustment trade payables and accrued liabilities										0.6
Environmental liabilities			$ 3.2						3.2	3.2
Revenues									1,145.6	1,145.6	2.1
Loss from continuing operations before income taxes									$ 23.3	$ (40.8)	$ (8.5)
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%
Real Alloy Acquisition | Customer Relationships
Business Acquisition [Line Items]
Amortization period										7 years
Real Alloy Acquisition | Consideration paid at closing
Business Acquisition [Line Items]
Payments to acquire businesses gross excluding escrow deposit		$ 496.2