Business combinations - Schedule of Purchase Price Allocation (Detail) - USD ($) $ in Millions				3 Months Ended
	Nov. 01, 2016	Sep. 03, 2015	Feb. 27, 2015	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015
Liabilities:
Goodwill					$ 42.2	$ 104.3
Beck Aluminum International, LLC (“Beck Trading”)
Purchase consideration:
Cash paid at closing	$ 23.6
Total purchase consideration	23.6
Assets:
Inventories	10.6				$ 10.6
Property, plant and equipment	6.8
Equity method investment	6.1
Prepaid expenses, supplies and other current assets	0.1
Total assets	23.6
Liabilities:
Total purchase consideration	$ 23.6
Real Alloy Acquisition
Purchase consideration:
Cash paid at closing			$ 501.2
Initial working capital adjustment				$ 31.3
Final working capital adjustment		$ 2.4
Total purchase consideration			554.5
Assets:
Cash			10.2
Trade accounts receivable			150.1
Inventories			157.7			6.4
Property, plant and equipment			311.7
Deferred income taxes			0.7
Prepaid expenses, supplies and other current assets			19.9
Identifiable intangible assets			17.0
Total assets			667.3
Liabilities:
Trade payables			112.4
Accrued liabilities			26.9
Accrued pension benefits			46.0
Environmental liabilities			12.1
Noncurrent asset retirement obligations			5.9
Deferred income taxes			7.5
Other			6.6
Total liabilities			217.4
Estimated fair value of net assets acquired			449.9
Total purchase consideration			554.5
Estimated fair value of net assets acquired			449.9
Goodwill			104.6			$ 104.6
Real Alloy Acquisition | Redeemable Preferred Stock
Purchase consideration:
Fair value of Redeemable Preferred Stock issued			$ 19.6