Business combinations - Additional Information (Detail) - USD ($) $ in Millions				2 Months Ended	3 Months Ended								10 Months Ended	12 Months Ended
	Nov. 01, 2016	Sep. 03, 2015	Feb. 27, 2015	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
Business Acquisition [Line Items]
Business Combination, Acquisition Costs														$ 1.0	$ 14.8	$ 3.4
Revenues					$ 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														(103.8)	(40.8)	(8.5)
Loss of equity method investments														(1.1)	0.0	0.0
Goodwill				$ 42.2	42.2				104.3				$ 104.3	42.2	104.3
Acquisition of Real Alloy															104.6
Environmental liabilities				4.0	4.0				4.3				4.3	4.0	4.3
Total deferred tax assets				419.5	419.5				388.2				388.2	419.5	388.2
Total deferred tax liabilities				16.0	16.0				12.5				12.5	16.0	12.5
RANA
Business Acquisition [Line Items]
Goodwill				33.6	33.6				95.4				95.4	33.6	95.4
Acquisition of Real Alloy															95.4
RAEU
Business Acquisition [Line Items]
Goodwill				8.6	8.6				8.9				8.9	8.6	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
Transition Service Agreement
Business Acquisition [Line Items]
Services expense														$ 0.9	7.4
Minimum | Transition Service Agreement
Business Acquisition [Line Items]
Service period														3 months
Maximum | Transition Service Agreement
Business Acquisition [Line Items]
Service period														24 months
Real Alloy
Business Acquisition [Line Items]
Accounts receivable due from Beck Trading				6.8	6.8									$ 6.8
Trade payables due to Beck Trading				0.5	0.5									0.5
Goodwill				104.6	104.6									104.6
Beck Aluminum International, LLC (“Beck Trading”)
Business Acquisition [Line Items]
Percentage of voting interests acquired	49.00%
Cash paid at closing	$ 23.6
Business Combination, Acquisition Costs														1.0
Inventories	$ 10.6			10.6	$ 10.6									10.6
Fair value adjustment to inventory														0.3
Distributions received														6.0
Revenues				10.5										1,249.7	1,145.6
Loss from continuing operations before income taxes				(1.1)										(103.2)	(31.7)
Loss of equity method investments				$ (1.1)
Real Alloy Acquisition
Business Acquisition [Line Items]
Percentage of voting interests acquired			100.00%
Cash paid at closing			$ 501.2
Business Combination, Acquisition Costs															14.8	3.4
Inventories			157.7						6.4				6.4		6.4
Fair value adjustment to inventory			4.2
Revenues													1,145.6		1,145.6	2.1
Loss from continuing operations before income taxes													(23.3)		(40.8)	$ (8.5)
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
Goodwill			104.6						104.6				104.6		$ 104.6
Fair value of trade accounts receivable			150.4
Fair value assumptions, discount rate															13.50%
Estimated economic useful life															20 years
Amortization of purchase accounting adjustments														$ 0.8	$ 5.6
Business acquisition purchase price allocation adjustment trade payables and accrued liabilities															0.6
Environmental liabilities									3.2				3.2		3.2
Business acquisition, goodwill, expected tax deductible amount									$ 66.3				$ 66.3		$ 66.3
Real Alloy Acquisition | Customer Relationships
Business Acquisition [Line Items]
Amortization period															7 years
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 | Consideration paid at closing
Business Acquisition [Line Items]
Payments to acquire businesses gross excluding escrow deposit			$ 496.2